Sweetie it is not a matter of being 'right' or 'wrong.'
Your parents are still in the 'throes' of codependency when it comes to your sister. Some day, don't know when, they will finally reach their breaking point.
For now, they just do not understand, that by not 'helping' your sister it is helping your sister.
Please do not feel guilty, I know that is hard. You have your own family to worry about and care for, a home, husband and children. Those are YOUR PRIORITIES.
I would suggest however, to spare some time and get to some Alanon meetings. They will help you so much, not only in dealing with your sister, but in setting boundaries, and sticking to them and not feeling guilty about it.
Just know that your parents are going through a very hard time right now, and haven't reached the point of "ENOUGH IS ENOUGH."
Please keep posting and venting and let us know how YOU are doing, we really do care a lot.
Love and hugs,