Hi OP,
I will echo a lot of what other posters have said: start as you mean to go on. And this applies not just to your relationship with your future MIL, but also to your relationship with your partner.
Take a step back and consider what you want and what you are prepared to tolerate, both in terms of the wedding and your relationship with him.
The wedding is only a day but everything that happens in the run-up to it is like a magnifying glass of whatever problems you will encounter in your relationships with your partner and his mother.
If you had a rational adult discussion with your partner once you two decided to get married and came to the conclusion that you just wanted parents, grand-parents, siblings and nieces/ nephews, then I don't care what justification he comes up with to side with his mother, he owes you an explanation for his change of mind. And if the only thing he can come up with is that he's trying to keep his mum happy, then you both need to realise there's something wrong with this picture. Because the only woman he needs to worry about here is you.
I second what previous posters said: consider very carefully whether you want to enter into a married relationship with a man who defaults back to 1) traditional views about a woman's role that don't match your won, 2) doesn't support you 100%.
Unfortunately, I speak from experience.
By the time we got married, we'd been together for 6 years, engaged for 3. Setting the wheels in motion for the wedding made my now MIL behave in a completely bizarre fashion. I was worried my family was going to be an issue, but in fact my MIL turned out to be a complete nightmare and my mum was awesome. I wanted a small wedding with GPs, parents, siblings and nieces/ nephews like you. That's 0 GPs on my side (as my grandma was unwell and couldn't travel that far), my mum as my dad is dead, my 2 sisters and their husbands and 8 kids in total (flower girls and page boys - thank God we don't do the bridesmaid thing in France, that saved me an argument about my SIL). On my husband's side: his grandma, his mum and step-dad, his dad and step-mum, his sister and boyfriend at the time.
My mum gave us a budget on a par with my sisters' large weddings to spend as we wished, despite the fact that it was in the UK where none of her friends came. The only thing she asked for was for my auntie on my dad's side to be invited to the wedding as she was the only relation who bothered to make the trip, and I'm glad I said yes.
On the other hand, my MIL imposed that we hold a reception in the evening for her mates (as a pay-back for the weddings that she had been invited to), she and my husband's step-dad only paid for 2 friends of theirs to sing during the drinks do before the buffet, and that was that. They didn't give us the money to spend on something useful like the food or flowers. Oh no. And the reception was basically paid for by my mum, with maybe 15-20 people attending on my side out of 100.
My FIL threatened not to come to the wedding because we didn't invite his wife's children, whom I hadn't even met. (Trying to put his wife first? can I blame him?) But thank God, my husband's grandma must have said something and he and his wife did come, but that did put a strain on the relationship.
We had the evening reception and it was OK. I didn't have the wedding I wanted and I realise I still resent it 12 years down the line (which is stupid, I really need to move on!). The fall-out came as a surprise because until then I'd got on with MIL really well. But you can see some warning signs so I hope you will act on them.
Other posters have said it won't stop here and they are right. When we found out I was expecting, MIL and step-dad made it all about them becoming grand-parents and treated me like a vessel. After we got out of hospital, they came to visit, saw the baby whilst I was in the shower trying to make myself decent for them and left without seeing or congratulating me after a difficult birth.
I'm now mum to 2 boys and I hope I won't be the MIL from hell when they get married. I have 2 sisters but I only get on with one who we have appointed as guardian, so if the size of the wedding allows, I'd like her to be there.
I'm also auntie to 9 children. I'm only godmother to one of them. If they invite me, I'll be happy but, if they don't, I don't think I'll be offended. We live far and don't see them much. It will be their wedding and I hope in this day and age they will have a chance to do what they want. (Your MIL may not have had the day she wanted for herself but that's not your problem.)
If your partner is worth it, OK, but brace yourself.