For four reasons, 1st of all it would fuck with Riser, 2nd it would fuck with Issei, third, it would fuck up canon. 4th, she was going to be a hottie and while she may be his sister, she wasn't his sibling in his mind.
Anime & Comics · A_Real_Werewolf234
"...Alright, Mommy. But don't be sad, okay? I'm growing up a little, but Mommy is still Mommy. Nobody can replace you, okay?"
Life in Vain: Jobless Reincarnation (Mushoku Tensei)
Anime & Comics · HappyVainGlory