Scarlet-Fernandez

Yeah, it is kinda illogical.

First, Wendy is not their child, even if some part of the fandom, ME one of them, have that headcanon, of Wendy being their adoptive child, it is just a headcanon.

Second; Justin is not canon/official yet, sensei was really clear with that; so, he is not taking awat anything from anyone, and even if he was canon, being love by their parents is something a child deserves, especially if your parents are Jellal and Erza, two people that really know how is to be a child without family.

Third; no matter how many children they would have, Jellal and Erza are loyal with their feelings, they have love each other since childhood, so, they are not the type to stop caring from someone important to them like Wendy, especially Erza, since she invited Wendy to join FT, she has always feel responsible of her, and their relationship is like a big sis/little sis, and FOR ME, motherly.

Fourth, Wendy is like 13/14 right now, and sensei draw Justin as a 10 year old boy, so, even if Jellal and Erza have him right now (and we know it would take a lot more lol), Wendy would be old enough to be a proper big sis for him, at his 10yo she will be 23/24 at least, what can possible take away a child from her, an adult?

Fift, Wendy is the sweetest child, she will be so happy for them, and will care, protect and love Justin, she probably will teach him a lot, like how Irene (Justin Grand mother) is teaching her. She will love the child of the people that take care of her when she was a child too.