User blog comment:B214/Volume 23 discussion blog/@comment-201.19.100.21-20170409030136/@comment-3965848-20170410085124

@Ultimate Pawn I'm aware their personalities are different, I already noted about role reversal part, I only pointed out the similarities in appearance, abilities and their roles in the story and that's where the line is drawn.

For the record, it's not exactly incest since he's not blood related to them. Like Issei's parents, the MC's father brought in the girls into his household when they had no place to go, he may see them as daughter figures but is OK with his sons relationship with them and having a harem. So it's a similar scenario with Issei, his parents, Asia and all the girls as you described.

I see them more like roommates rather then siblings and maybe a real couple later on, which she'll probably stop calling him "Onii-chan" at the time.