I think if you don't set some boundaries for MIL now you will regeret it in the future. Do you see your DP's ex?? Do you have a relationship with her? if no, then of course she shouldn't be invited, this is a party for YOU not your MIL. i would be scared off by the invites alone, yes it is your grandchild MIL but its first and foremost your son's and his partner's.
When I have had a baby shower (i have had 2 LOL) the friends who organised it asked ME to give them the guest list and they invited those people, it wasn't about who THEY wanted.
Heaven help you if you guys get married LOL They'll fill the wedding with their friends.