Welcome! I have a dsd who is 6 years older than dd and 9 years older than ds, so I know how it goes trying to keep a wide age range playing happily :)
We are lucky that dsd still enjoys playing pretend and imaginary games-maybe not quite hte same way the younger ones do, but it stills omething they can all do together happily. Otherwise, try to find activities that they can all do, like arts and crafts or listening to a story, etc., even if they do not all do it the same way, and hopefully that will make them realize they do have a little in common!
I dont' know how often your dd sees your dsd, but I know it took dsd much longer to get used to having a younger sibling than it would have if she saw her every day. I think that older kids tend to take a while to see the littles ones as actual people too, if you know what I mean- 3 is just about the age where they can actually play a game or follow directions without "messing up" the older one's play, so hopefully as your dd gets older, your dsd will come to enjoy spending time with her and see her as a sibling/playmate instead of "a baby." I don't know how long you have been living together, but this can be a big adjustment to make!
As far as listening...I hear lots of people say to leave all the discipline to the dad, that you have no place in that, etc., but that has not been practical for us. I am the stay at home/work at home parent, so it naturally falls to me to discipline when necsesary if we are alone together. No way am I going to be the parent saying, you just wait till your dad gets home! If there is a big issue, I do ask dp deal with it for exactly the reason you said-I don't want to be the nagging stepparent/bad guy all the time (even though I still often feel that I am). Otherwise, if she is an overall good kid like you say, try not to sweat the small stuff. Realize that stuff she does probably annoys you more than it might if it were your dd doing it, that you both have little time to get used to each other before you are apart again, and try to enjoy it. Easy to say, hard to do, but I hope that helps!