Edited & removed! Thanks!
You're reading too much into it. It's not your problem if they're wearing similar outfits! No one will notice or remember! I could barely tell you what my own mother and MIL wore to my wedding! It seems like the biggest thing in the world at the time but it's really not.
I'd be upset too. Before saying anything to MIL, I'd chat to your mum and see how she feels about it.
What does your fiancé think?
Are they so alike that they'll look their they're wearing the same dress or will they just complement each other well in photos?
Perhaps they'll style them differently with accessories?
Hi there, isnt it funny how us brides overthink everything