Awkward hotel arrangements

"how can I ask her nicely to put her family elsewhere and let me have the rooms for my friends?"

I'm a bit confused by this - is your MIL paying for the venue? Because if not, then surely you and your OH are the ones who get to decide who stays there and who doesn't. All you have to do is assign rooms to those you want to be there.

If you have reserved the rooms already and people need to book through you, then just tell MIL that there are no spare rooms once the wedding party has been accommodated, and that they need to be staying because of their roles.

If you haven't reserved the rooms/people need to contact the hotel direct to book, then you need to get the people you do want to book rooms to do so asap - because it's going to be very awkward to undo the booking once it's been made.

If MIL has already booked the rooms, then you'll just need to appeal to her good nature. It might be best if OH does this, pointing out that your bridesmaids will have been working so hard to support you on the day and really need rooms on site where they can rest after their hard work.

Just be upfront and say you would prefer your bridesmaids to be with you on this occasion. Best of luck its your special day you should have what you want. Hope your Mum can understand that!