15 Reasons Why Erika's Sister-Relationship with Mairu is a Bad Idea:
Erika insists on giving only the best advice to her little sister. But her advice actually sucks and shouldn't be taken.
Erika has no prior experience and her knowledge is based on literary sister relationships which aren't realistically obtainable.
Erika is really impressionable and it might end up with Mairu being the “big sister” and Erika taking advice from HER.
If Mairu gets close to Erika she'd also have to get close to Walker and that's never good for anyone.
Erika has a large collection of hentai and she loves to share.
Erika doesn't believe in a “bed time” and would encourage all-nighter sister sleepovers, which makes for a bad school day.
Erika has a loose moral code when it comes to violence and would in fact support Mairu's decision to beat up bad guys. Some might consider this to be wrong.
Erika has always wanted a sister and would want to give her everything she wanted, even if such a thing fell under inappropriate categories.
Kyohei already has 3 children and he doesn't need another one. Please think of Kyohei.
Erika's apartment walls are thin and her neighbors are tired of listening to her talk about BL and penises, so they definitely don't need more of that.
Erika loves to give relationship advice so any of Mairu's love interests are in for a rough time.
Erika is an adult and can get into all kinds of R-Rated places.
Erika is a master cosplayer and would only encourage Mairu's outlandish, exaggerated behavior for the sake of playing a role.
Erika is only a child herself and is therefore not properly qualified to care for the moral upbringing of another child.
Erika does not eat a well balanced diet, so neither will Mairu.