Thanks ladies. It would be much easier to just invite everyone and if she has a problem with it that's on her for not responding.
I would do an all in one and let the mil know the situation and keep some extra invites in case she wants to invite ppl y'all haven't considered yet.
I would if I was you just in case. If ahe makes a deal of it you can say well you didn't respond and we needed one so we just thought the best thing was for everyone to be invited. If she gets huffy about that at least you are covered and people will see you considered everyone. Maybe tell her one last time you need to know right now because the invites are about to go out, but honestly if she is difficult I would just invite everyone anyway.