I have the best of families in my opinion and I still would not share renting with a family member. its too easy for family to take advantage of each other, too easy for one or the other to rely on the other for rent, utilities...

my point if this was me I would be honest with the parents, that I prefer something where I wouldnt have to depend upon a sibling for their portions of the bills, and they depending on my portion of the bills. anything can happen on either side that can result in bills not getting paid or what every.

then I would be looking into places that I could afford on my own, even if it meant living in a hotel or motel or BnB temporarily. and let the more unstable sibling have the family home so that they can get work done on them selves.