It’s unclear to me whether @mnemosyne’s “them” was referring to the parents, the sib, or all three collectively.
It’s also unclear how wacky the parents would or would not be if they didn’t have the sib in their household. Nor whether there’s any alternative to that dangerous and destructive situation.
But if the parents are becoming equally toxic due to the influence of the sib, well, now there’s an even larger decision to make.
Here is a recent thread about a very different but equally difficult family dynamic.Where people of good heart are dealing with a bad situation with difficult relatives and are now struggling to find a way forward.
The details are utterly different than @mnemosyne’s situation, but at a high enough level the problems of family are eternal and universal. This thread might say something that resonates with them.