Am I not being stubborn enough?

For a little background my Fiancé and I are getting married in June of 2019. And we’re getting a little of backlash from his parents. They feel like we aren’t telling them anything and letting them help with any when there hasn’t been any big wedding planning happening. I partially don’t even want them at the wedding because they don’t like anything we’ve chosen or have already paid for. They have said multiple times they don’t want the wedding to happen anymore because no one is telling them anything. They didn’t like our venue because they weren’t able to come and look at when I think it should just be my fiancé and mine decision (and my parents since they’re paying). My parents paid for 150 people to be there and to up the amount to 225 it’ll be an extra like 2,000. They don’t like the caterer I chose. It’s something I’ve had at work, its literally just bbq steak and chicken and other fixings. They want to do a pig roast which I think is tacky. Now we still haven’t gotten the save the dates out because his mom thinks she’s getting 75 people to invite and she isn’t listening to anything we say. If we let her have 75 people, after the wedding party, that’ll leave 2 people for my parents to invite and we don’t get anyone. Her reasoning for them getting 75 invites is because they won’t know anyone there and they don’t like how they’re being told how many everyone is getting.

I don’t know how to be strong enough to tell her to back down and just suck it up and let us plan our wedding when she’s acting like this. She’s a very stubborn and hard headed woman and very brash.