I swear if the siblings turn out to be unrelated...
My theory now is Tsukuyomi is his actual blood related sister, probably not but that'd be interesting.
Too many romcom adaptations from LN. Honestly, I hate them. I prefer manga originals, at least there is a new chapter every week. After a month (or even more, since sometimes scanlation groups don't update for months) I usually forget everything.
Weird flood of brother/sister manga on the site lately, what are you trying to tell us, Japan?
This one is a little weird, but it's more tolerable than the last couple I looked at. Early intro of a love interest for the male MC helps it, I think.