I actually really liked this chapter. You can tell how troubled Ashima is because she doesn't feel like she belongs anywhere, and she messes with the sister because she doesn't feel like anyone cares about her expect Rin. I get that they are blood related, but it's really hard to see each other as family when you're treated so differently from the rest of the group, hence why I don't even mind the subtle incest because Ashima doesn't see how she belongs to anyone, considering her own mother abandoned her.