You said, "There is no visitation order in place as it is not going to court."
What do you mean by that?
I agree with Lauren--you need to get a hold of somebody, that is too much for you and your husband to have in your laps esp. without a plan. It isn't healthy or helpful for anybody involved in the situation, including and especially those kids.
We were in a slightly similar situation as volunteer temporary custodians for the children of somebody we knew. In our case, it was not a relative but it was a similar situation in that the kids were removed by CPS but then we volunteered to take them and were given temp. custody.
First, I will just point out that it seemed to me that it is very easy for kids in kinship care/temporary custody situations to get the ball dropped on them somewhat by CPS, even more than kids in foster care, as CPS is not the one who actually has legal custody. They get the kids out of the dangerous environment, put them in a safer environment with you, hand the reins and legal custody over to you. It is easier to put stuff off onto you because their requirements are different for kids in foster care where the state has custody and there are more defined rules/regs for foster care than kinship. (At least in my state.) So that makes it hard in some ways--also, I discovered that the CPS workers didn't always know exactly what the rules WERE in our situation--because the kids were not actually in foster care. For example, what kinds of medical care we were or were not legally able to authorize and what required the parents to be consulted, legal timetables are different, etc. That part gets very tricky. All that to say, you really have to advocate for the kids. And for yourself in all of that too.
Practical hints on dealing with CPS--I recommend emailing social workers etc., rather than calling, because then you have automatic documentation of the fact that you have made repeated attempts to contact them, and the content of your emails become part of the case file. (It is easier for them to ignore phone calls than emails because there isn't the same paper trail/no proof you tried to contact them. Also, I think a lot of times the social workers prefer emails anyway, in that they can deal with them when convenient and it doesn't take as much of their time as a phone call.) Email makes it easier to deal with multiple people too--if needed you can cc emails to other people (counselors, attorneys, teachers, whoever) when it is necessary and appropriate. As a side note, I got a separate email account shared between me and my husband that I only used for communications related to the kids/their case. That made it a lot easier because then we both had access to all the email related to the kids, and it was all in one place. Go to any hearings, case conferences, etc., regarding the kids and their case. Even if you don't get into the courtroom, showing up helps.
For us, there was a court order that visitation was unsupervised, short term, no overnights, to be scheduled by CPS. When we first got the kids--things were a bit hazy. CPS was kind of like, "it's up to you to say what works for your schedule, but generally we like to see one hour per week of visitation to start out with." Meanwhile, the parent thought that because CPS was giving us some discretion about visitation, it was all on us and we could do whatever we wanted, and that if s/he could just pressure us enough, s/he would get whatever s/he wanted in terms of visitation (which would have been an all day every day thing) and would get to make all the decisions about stuff for the kids but us bearing the ultimate responsibility as the guardians. We got pushed, and pushed, and pushed on all kinds of matters and our own family rules and values were getting disrespected--finally we had to get really clear and set some strong boundaries. Not because I wanted to be mean or was trying to disrupt the relationship between the parents/their kids, but because that was what needed to happen for everybody's sake not the least of which was to keep myself and my life sane enough for me to be able to take care of everybody. Sounds like you are in that same situation, where the parents are pressuring you and trying to take advantage of a new caseworker to push past your boundaries and get what they want. I am sure that it is even harder because it is your sister and so you have that relationship going on too.
So, what I am going to say is going to sound harsh, but here it is: you are not the babysitter in this situation. Your sister/her boyfriend have not hired you and they don't get to call all the shots here. You are the legal custodian and you are the one responsible for taking care of the children and doing what is in their best interests at this time. Through their own actions, your sister/her BF have lost the opportunity to parent their own children at this time and the state had to step in for the safety and well-being of the kids and you stepped up to care for/parent them until their own parents regain that ability. They are lucky that you have done so and that their kids are not in foster homes.
If your sister/her bf want the chance to see and parent their kids all day/every day, they have to step up and change whatever conditions resulted in the kids being taken away and jump through the necessary hoops to get their kids back. At this point they haven't demonstrated that they have done so, hence the kids are with you. If they have unrestricted access to the kids on their own terms but no responsibility for the actual day to day work and responsibility of caring for them, that is the worst possible set of conditions for everybody. They get to sort of have their cake and eat it too and there is no real incentive for them to step up and put in the effort to do whatever it is that they need to do to get their kids back, because they get to have all the fun parts of parenting but none of the crappy parts of parenting. It's not good for the kids because instead of getting their own crap together to be better parents to their children, your sister/her BF are ultimately going to end up just moving their dysfunction on over to YOUR house. Meanwhile, it puts an undue burden on you/your husband, because you get to do all the crap parts of parenting those kids, namely, being a responsible adult who does laundry and cooks meals and helps with homework and enforces bedtimes and takes them to doctor's appointments and all the other not-so-fun stuff. That all-the-time visitation situation at this stage of the game would seriously impede your own family life, and your own children as well as your niece/nephew--they all need stability, consistency, and to be in an emotionally healthy environment. (Once the kids are moving toward going home--different story and appropriate to ramp up the visitation then.) It sounds like it already is impeding that with the current visitation schedule--that is a very long time period to be responsible for supervising somebody's interactions with their children and to have somebody over at your house or you be out with them, and having a big scene and an argument at every visit is NOT okay--that is basically creating a situation of emotional abuse/domestic violence at YOUR house that everybody is getting exposed to. You need to set healthy and appropriate boundaries (easier said than done, I know!) and enforce them. I would draw a line in the sand based on what you think is best for those kids and you/your family. That may very well be different from what your sister/her BF want or think is best, but remember that their best thinking got their kids in this situation in the first place. Let your sister/her BF talk to the caseworker or judge if they want it changed, and let the caseworker call you to discuss it and let them advocate for more liberal visitation. You could maybe change the schedule if it works for everybody to have one longer visit on one of the days of the weekend and then a shorter visit mid-week--that way, it isn't like they are together all the time on the weekend and then don't see each other for the whole week. Over time, if your sister/her BF are doing what they are supposed to be doing with their case plan, they will get more visitation with less restrictions. If they are screwing up, they will end up getting less visitation/more restrictions. Also, I would definitely try to put the kibosh on the big scenes/fighting--I would make it very clear that you are no longer going to tolerate that. If you have to ask her to leave or call the police if things get out of hand, it may come to that.
I know all of this is very hard--I know that I sound like a total hard-ass here but really, I cried a LOT. I felt terrible for the kids to be without their parents, I felt terrible for the parents to be without their kids, I hated being in conflict, I had a hard time learning to set good boundaries, and I had sort of "survivor guilt" that I was getting to parent my kids and theirs. I really had to tell myself that enabling somebody else's unhealthy behavior wasn't helping them, and setting good boundaries was actually a kindness to myself and everybody else and was the best thing I could do for all involved. I would seriously recommend finding a good therapist and trying to find a support group of kinship caregivers and/or foster/adoptive parents.
Hope some of that may be helpful . . . Good luck!