Can you tell your mil she can't get ready with you?

I'm looking for advice on how to politely tell my MIL that she can't get ready with my bridal party in the morning.

My fiance and I have been together for a long time (almost 10 years, engaged for 2) and our wedding is this October. When we began wedding planning my fiance's step mom came to me and told me how she planned to get ready with the girls and would like to get her hair and make up done if that's alright with me and at the time, not knowing what to say, said that the salon is very small (which it is) and since we have such a large bridal party I would have to see if they could accommodate more with the time frame we had. I have 7 bridesmaids, myself, two flower girls and my mom getting ready with me that day. She has not approached me about it since but now that we are coming to be a few months out I figured I should revisit the question.

Now this is my fiance's step mom. My fiance also has is real mom who will be flying in from Washington. The two do not always get along and both do not have any relationship with my mom. I would love to include everyone in an ideal world but feel like having them there would cause unneeded stress and tension and would be rushing the hair stylist and make up artist. Plus I just feel like I want to spend that sentimental time with my mom before I get married.

Neither of the mother in laws is helping to pay for the shower or wedding. The step mother in law offered to do address labels for my shower invites and to make a couple baskets for the shower. My mom paid for the bridal shower herself.

I spoke with my fiance about it and he said he agrees with me and there is nothing wrong with it just being the bridal party and my mom that day.. I just don't know how to go about it with out hurting any feelings. Looking for suggestions as well as advice on the situation.

Thank you in advance!