Based on recent pic I have two theories regarding the girls with Pietro
According to Peters and EMPIRE, Magneto has never met Pietro, he may not even know he produced a child(ren).
Magnetos name is Lensherr in the movies, yet there was a pic of a home with a Maximoff mailbox.
Theory #1
Pietro is an orphan who lives in a foster home of people with the surname Maximoff and these two children in the pic are other foster children.
Theory #2
Pietro could live with his Mother (Maximoff) and the two girls are his sisters. Neither are Magnetos children as he'd have to have impregnated the woman (with Pietro) then came back years later, been blind to her having a child and impregnated her twice more.