The way we split up the rent was what we agreed on since we are sharing the same amount of "personal" space - she has one bedroom and all three (will be 4 in the fall) of us have the other and we have a shared common space. We do split the utility bills to where we pay more since we use more electricity, water, etc. I let her be the one to decide all of this. I am the type of person who is deathly afraid to step on anyone's toes, so I pretty much let her call the shots on how everything was going to go.
To clarify what I meant about my parents, I meant that more people will be affected than just us, and my parents will not put up with it as we have.
Oh my. This is a recipe for disaster.
Your parents still have no say whatsoever. Just because they are paying for sis's share, their name is not on the lease. And I am sure roommate knows this. Mom and dad can say all they want to, does not mean roommate has to listen or will listen.
You are having a baby. Time to stop going to or expecting mom and dad to take care of your problems. (although, I am a parent and I help my children as much as I am able to do so, but in a case like this, they make their bed, their choice, they must learn to deal with the consequences of their actions or in actions).
You need to buck up and step on toes. Where is your dh in all of this? He can not say anything? IF roommate owes you money now, and cannot afford to pay you now, she is not going to be able to at a new place.
My advice, look for a new place for your family and sister, (because I don't think you could stay at this one if you have no spine to tell roommate she will have to find a new place) and tell roommate that she will have to find her own place. Rooming together has not worked and you don't see it working in the future at a new place either. Your family cannot continue to support her. And I think you will probably have to count any money she owes you as a loss.
You don't say how she is working on repaying what she owes you? Is she giving you extra? Is she giving you anything at all towards repayment? Or is she just letting things slide? If she is honestly trying to repay what she owes and making current payments, then I might consider finding a new place with her, if she can pay her fair share without borrowing anything else to move into new place.
And no way would I live in current place with a family and a sister in one bedroom. Need to find a new place, (if including roommate) with three bedrooms. And you are not using that much more utilities than she is, so I can see paying a bit more than everyone else, but not a substantial amount.