Teenage preg. Advice

Okay so this is a long one and i need to know if I am in the right to back out and how you would handle it.

My sister (15) ran from home, slept with a guy (16) and got dad to pick her up later that night not telling them. They needed space to cool off and agreed for me to take her in for a week (23 with baby at home) anyway when she gets to mine she says that she had sex and that it wasn’t the first time (she had the implanon in 1 week ago) parents found out through her phone and well werent happy. We got her the morning after pill and waited. A week went by and yhe day she was meant to go home i said to do a test to know (me thinking itll be neg) but NOPE positive. Another whirl wind of a time. FYI i am nearly in my third trimester and this is bloody stressful. My parent want me to take her to her appointments as they dont want anything to do woth it but this is starting to hit me hard. Here i am heavily pregnant and she is trying to terminate. I am pro choice all the way but i cant help but feel gutted by it as i still see it as a baby (i know its not but its how i feel) and i feel like i need to step back otherwise im goibg to get to broke up about it and cause too much stress. I want to be there for my family but i need to put myself first. Am I in the right to do that or should my sister be first as she needs support people too right now and my pare ts are struggling to do that.

EDIT:

Thank you for your support guys. Our parents have stepped up now and are taking over the situation.