First of all, why would your parents put that kind of burden on you as a 17 year old. That's really not fair of them to expect YOU to raise him. You've got your whole life ahead of you. To be tied down to a child is just unacceptable, especially since he's not yours.

I think your parents need to intervene on the behalf of their grandson. Someone needs to stand up for this child since he can't very well speak for himself. They should also get your sister psychological help. Sounds like a strong case of post partum depression.

I wish you alot of luck. Please keep us posted. You'll be in my thoughts and prayers.