@Nep
Rather, the only one who knows of the abuse is Hari, it's not like she has to outright say anything at all. There's nothing stopping her from saying "I'd like to stay with the Vastias for a time, I think it would help to take my mind off of recent events". It's not like those twins or their parents have a reason to refuse, what with the whole group going out of their way to assure two way communication just so they could speak to her.
To them, her arrival would be a two way blessing, and one less child to be stressed over for Eugene as well. Beyond that, Hari is the only one suffering blatant physical abuse. Cabel was just at the Shuman residence himself, and that ended up giving Eugene less worry knowing he was being healed and looked after.
Furthermore, I think the avenue the aunt is going for centers more around adoption. Eugene is, without a doubt, the heir. It's unshakeable fact at this point and the aunt isn't so foolish as to neglect that. Rather, having an Ernst heir, blood related or not, grants her power. The large changes thus far were the Vastias arrival date and the death of the Ernsts, and although Erich was never adopted in the other run, despite his disposition towards Hari, there's a chance that avenue will open up with the slightly closer relationship. It might sound awful, but she dotes on him for a reason. Perhaps she wants his share of inheritance, a likely route, but an easily discernible one.
but she doesn't actively alienate the children from eachother, just treats them differently in closed environments, so I believe her efforts are more disposition based towards herself. It's obvious she wants Erich to like her not in spite of, but rather than Hari or Cabel. It's not like she explicitly attacks Cabel either, and rather shows alot more emotion in regards to Hari. Perhaps Cabel isn't a threat or a concern. Regardless, one of the best paths is to leave negative subjectivity and come back with an objective mind for the confrontation. If the aunt's objective is Hari's amity, then a neutral state of mind will be all the more beneficial.