As for why she didn't help the Chosokabes after Chiaki's death, that hasn't been answered yet, but I will point out a few things:
1. This is a problem no matter who the friend is
I agree, it's a problem no matter who it was, but I think it's a
bigger problem if it's Tsubasa considering she's currently being moved to tears by their circumstances and still trying to get Chika to move into the manor. Yes Chika is stubborn but imo a recently orphaned middle school Chika taking care of her sister alone would probably be a lot easier to pressure into accepting direct help than a teenage Chika who's been holding down that position for years and has Yumi and, to a lesser degree Akira, around to help (Actually Yumi might have been around back then? I'm not sure. I think I remember Rin mentioning something about Yumi in middle school but I can't remember when or if that was actually said)
imo if it is Tsubasa, which I'm not saying it isn't, mind you, it opens up a lot of questions that I don't think can really be brushed aside by reset time stuff.