mog/mob dresses

How does your Mom feel about it? In my opinion, that's the real question. If both the MoG and the MoB don't mind wearing the same dress, then you should let it go. Afterall, they will be wearing it. If your Mom is upset, then I would convey this to your future MiL and ask her to return. Esp. since your mom can no longer exchange hers.

Its kinda neat that they both picked the exact same outfit. How often does that happen? Both your moms have the same tastes - hopefully that's a sign of more harmonies to come! :)