Ok, self, there is a time and a place for everything. There is a time to be polite, and a time to be honest, and now is the time for honesty.
So the next time your future mother in law says “I know you’re not inviting aunts and uncles to your wedding, but how about inviting my sisters and brother and making them pay their own way”, you don’t answer “Oh, the venue is kind of small” or “But my aunts and uncles might get offended”, you answer “Because I don’t want them there. Nothing against them, I just want a small wedding”. The other answers may be sort of true, but the last one is the whole truth. You don’t need to get into how tacky it is to ask wedding guests to pay for their own meal - that honesty is probably best supressed.
And when she starts again on “Why don’t you and the bridesmaids carry plastic flowers?”, you don’t say “Oh, I’ll think about that”. You can answer “No, I want fresh flowers.” You can even add “I like them better”. You don’t have to pretend to listen to her idea just because it will save money. She’s not paying, you are, and you want real flowers.
If you’re feeling daring, and she’s still nagging that her sister wants to see you get married, you can say “I’ve told you she’s welcome to attend the ceremony, just not the reception. Does she want to see me get married, or does she want a free feed?”
Other than your revolting habit of hiding the truth behind a veil of politeness, you’re doing well Cazzle. This has been an incredibly stress-free experience since you decided to cut out the bulk of your relatives and have the small intimate wedding you’ve dreamed of. Don’t let anyone try to bully you into doing something you don’t want to!
One lie leads to another as you found during the whole “Invite my siblings” “Ummm… venue is too small” “Hire this venue instead” “Ahhh… your son doesn’t want to because he says it’s a dive” conversation. Tell her “Dear Mother-In-Law-To-Be, we are having these guests on this date at this location because that’s what we want. We’re wearing these clothes, carrying these flowers and hiring these people to cater because it’s up to us. Please, be there and have a good time, but don’t think you can tell us what to do. This is our day”.
Most especially, don’t feel bad for rejecting her idea to hold the reception in the middle of the bush, 40 minutes drive from where you’re to be married, three hours before the wedding is due to start, as many people would find the idea of having the reception first laughable, not just you. And I don’t think you’re wrong to be offended because her idea didn’t include inviting your family, just hers.