Your parents are using her disability as an excuse. I’m very sorry you’re dealing with this. I’m angry for you just reading your post. It’s bullshit.

As other have pointed out 7/8 year olds know the difference between right and wrong. You shouldn’t have to give in to any demand from your step sister.

You should not pay the hospital bill.

They don’t realize it now but they are building a lot of resentment in you.

As others have also suggested, talk to an adult you can trust or speak with someone from your school whom you believe can be trusted not to bring this back to your parents.

Are there any relatives you can stay with for a little while?

This isn’t fair to you. You’ve worked hard for the things you’ve both. It’s not as easy as your parents have made it or to seem to just get up and buy them over. You bought them for a reason.