Gyari the rock devi-dol is an open lesbian and she stated it herself that gender doesn't thing for demons.
I believe Kerori also has a feeling for her but chooses to decline due to her feeling that Gyari will never have enough with her as an equal.
If I remember correctly, the sister is now in a VIP family and have higher social status than her brother. After the family divorced, I think they cut tie and do not publicly reveal their relationship. So, Kuze only refer to Yuki as his sister in secret now.
I have a way of distinguishing them. Hisamura (our MC) has a bang covering his forehead while Shigemoto (in-universe MC) has a split bang in the middle. I think he also has a mole near his eye.