>Erica sometimes talks about his dad, but to my knowledge has never brought up his mom.
>If you meet up with your dad, he says that he always knew or believed that magical girls existed (or so i've been told)
>You and Isabelle hung out when you were very young.
> There's an enemy that mimics party members's powers, and for everyone else it just takes some moves that they know, but for Erica it takes a move called "Immortal Tsar's curse" which is also one of Marianne's moves
It almost seems like...
But honestly all of these things can also be explained normally (except for the tsar's curse thing, that one's weird.)
maybe his mom just doesn't happen to come up in conversation. Or maybe he has brought her up.
Isabelle being Erica's sister is a pretty universally accepted theory. Some more things to add to the pile:
> Marianne says Erica looks like Evelynne (Isabelle's father)
> After getting face TF from Alice, everyone says Eri's face looks like Isabelle's (including Eri himself and Yvette, Isabelle's personal yandere), despite Alice claiming she did only minor adjustments
> Nele says Erica's "Magical Girl costume transformation box™" looks exactly like Isabelle's, just with a different color, despite the fact Wendy and not Isabelle gave it to him
> When in a conversation with Isabelle Eri jokes about them "jokingly making marriage proposals", her reaction is such strong disgust that seems weird...unless that would be something like incest
> Erica's general skillset is similar to Marianne's. In fact, using Aurora Burst with Marianne in the party will cause her to react very suspiciously, like it's not a skill Eri is supposed to know.
> Eri believes that he had a brother who (along with his mother) died in some accident, which sounds awfully like genderbent version of the story Marianne tells about her own family
> In addition to the previous point, you can see in the sealed memory that Isabelle thought Eric was dead at some point.
Of course, there are alternative explanations. The one I myself use as a backup is that when Isabelle jumpstarted Erica's magic she (un)knowingly both gave him access to some of her family spells and it caused his body to naturally mimic Isabelle's.