I just read all of this and I wish I had better advice. I am a step mother, but I am lucky enough that I get to raise my step son full time. I wish his mother was more involved, but she isn't. Anyway, my advice is, your fathers wife knew ahead of time that marrying your father meant that he came along with children. It was her choice to keep having children with a man who already had a family that he had obligations to. Infact, I think at 16, you probably spend MORE time with your father then most children who live with that parent do. (I know at 16 I wanted to be out with friends or locked in my room doing homework). So my advice is do what you have been doing. it sounds like your father is a somewhat decent man and I hope he starts bringing the baby with him to the visits.
Or what about if you made a compromise and went from two days a week that are seperate to two days that are consecutive. Like if it is REALLY important to your dad to have you over night, do it, but skip the tuesdays so you have more study time.
I think it is great that you have a good relationship with your baby sister, but again, it was not your choice to have those kids and it wasn't your choice to have seperate families like you do.
For a 16 year old, you sound VERY mature, but honestly, you are a child and you should NEVER have to burden yourself with all of this. there should be no guilt trips on your and your fathers wife should remember you are a child as well and you deserve your down time as well as time with your father and sisters. Sounds like she is jealous because she knows you don't care for her. Well, her actions are what have caused that, NOT you being unreasonable.
Anyway, keep the lines of communitcation open with your father and your little sister, well, kids are boogers sometimes at that age, gotta love the, but they repeat everything they hear. Don't take it personally from her, just know that is sounds like her mother is very stressed and might be saying things she doesn't mean to.
in the mean time, keep up with school, keep spending time with dad and the little sisters and remember you are a young girl and deserve your freedom.