Angel yuri incest? Neat! Actually, if my seven years of Sunday school taught me anything, this makes some semblance of sense - angels are typically genderless (I think) and incest was fairly common back then (I think). I'm probably wrong though, so don't take my word for it.
It wouldn't really be supernatural otherwise. I have a feeling Kami-sama will let her return to Earth because her love is so pure or something like that.
At some points, (especially page 130) I was afraid that Mikoto was going to go into edgy yandere mode and either kill Oga-kun or kidnap onee-chan, but than I remembered who wrote this and felt bad for even doubting the greatly-talented Itou Hachi to not keep things optimistic and lighthearted, as they should be.
Kind of disappointing how little romantic attraction there is on Sayuri's part. There's a hint of it in the first chapter, then another possible hint in the 5th, but hardly anything in between. I get that this isn't something she's thought about before, but the barrier to her even considering giving in seems awfully high.
Honestly, I'm not a fan of incest but with Itou Hachi writing it, it doesn't seem wrong. Really loving this author!
Additionally, I read the comments and I'm glad I'm not the only one who thought of the possibility of her sister being dead. Anyways, it doesn't seem like that's the case YET.