hmmm I thought the sis was gay because in the first chapter wasn't she gonna do it with someone?
Oh well, maybe the author forgot after 2 years and 5 months

Well, first they just put on cosplay costumes and the 2 MCs thought that they would do "other things".
Second, dunno, the author just played with the "in love with her older sister" trope. You can interpretet it this way, that she backed out, because her older sister loved er as just a sister anyway.

This was short but really nice.

hmmm I thought the sis was gay because in the first chapter wasn't she gonna do it with someone?
Oh well, maybe the author forgot after 2 years and 5 months

No it was mistake from our lesbians, they have just doing cosplay, and Imouto-chan assist her friend with dressing into the costume.

my eyes hurt .

(I cannot believe this hasn't benn posted yet)

And they were roommates.

And they were roommates.

oh my god they were roommates