Basically what would happen if Eri didn't get sucked into the Gundam.

She'd grow up to be a rather jaded individual, since she and her mother know about what happened to her father and family all those years ago. Prospera would do some behind the scenes scheming to get Eri to be an "instructor" in the school, so that when Suletta finally comes around Prospera would effectively have two moles; one knowingly and the other unknowingly doing her bidding. But Eri is super protective of her little sister and tries her best to keep Suletta in the dark about what her and her mother are doing.

In fact Eri's super doting over Suletta and is trying her best to give Suletta a "normal" life despite all the subterfuge. Thus Eri as actually really supportive of Suletta and Miorine's marriage, despite the implications that this is all part of Prospera's plan. That said she never lets her little sister see her real feelings and keeps a neutral visage between her and Suletta; can't let people think she's giving her little sister any special treatment.

Eri also secretly works with Dawn of Fold, using them to "discreetly" deal with complications to the plan. And Eri is very effective at this; one of Dawn of Fold's best agents. Thus Sophie is smitten with her, wanting to be the little sister to Eri and is super jealous of Suletta because of that. Norea just thinks Sophie is weird.

Eri is constantly keeping an eye on things; both the plan and her little sister. She's hides around corners all the time watching over Suletta and while Suletta never seems to notice her doting, slightly psychotic, older sister... everyone else does.