The sister really doesn't deserve to be there. Like, the only reason she's there probably is because she ended up that fucked up over the fight and Lavi didn't have the heart to leave her hurt like that (or the trumpet girl from the adults' party took her there to help her heal).
I hope the author can have at least someone unwilling to have her there after they leave the onsen, because most people either aren't under proper context on the situation to understand the nuance, or are so close to it they cannot bring themselves to berate Conney for acting so destructively out of a distorted desire to protect her family through any means she considered necessary (which they understand more as a fault of the father than anything else).
Even if Conney was mentally unstable, nobody properly tried to help her as far as we were shown. At most they misguidedly enabled her abuse of Lavi thinking her intentions of protecting him were good enough an excuse for the kind of things she put him through. Someone HAS to point out that she should be separated from Lavi until she can properly reflect and apologize for all the pain she forced him through, even if Lavi is too good-natured to recognize they need that distance, even more so when he's a young boy still.