aunties, uncles and cousins should be based on the closeness of relationship you have
I was invited, as were my dc to my nieces wedding. It was a big wedding, and I’m pretty close to niece. Dc don’t see their cousin (my niece) much, but they were invited with partners . It included 2 nights stay and a long journey and whilst was a nice wedding, it all seemed a bit of a faff and stress for niece.
whereas her brother, my nephew, had much smaller wedding. And a bit last minute. I wasn’t invited nor were my dc. But bride invited her aunt. 1 grandmother included, but not the grandfather on other side. I didn’t mind, uncles on my side didn’t mind, and not aware grandparent who was excluded minded either. It was made clear that it was small wedding, and they were inviting those very closest to them only. Bride had very close relationship to aunt, and one grandparent was very involved in couple lives, the other Gp had never been involved grandparent
only about a year later I moved house, and have got a lot closer to nephew and his wife. See a lot of them and have a close realtionship with wife. We were talking about their wedding some time later, and I commented they did me a “favour” not inviting as it would have stressed me out. it was a “destination” type wedding in uk that was a long multi change rail journey and basic hostel accommodation …. I’m not good with sleeping bag type accommodation and complicated journeys 🤷♀️🤣
long and short, talk with people up front. Apologise, say you’d like to invite them and their partners, then Explain places are limited as you need to keep costs down to spend on (house, or something sensible you say even if not entirely true) . Explain you can only invite those you have the closest relationship with, and that you hope they’ll understand…and that you understand if they don’t want to come without their partner.
I’d make exceptions for partners where there is a need to travel together, that could be someone with a disability that needs support, or someone who literally won’t know anyone else and maybe will feel very uncomfortable being on their own. Or breast feeding mum, who needs dad there to manage baby etc. and partners where you socialise with them as a couple regularly.
other thing you can do, for older relatives like aunts and uncles that might feel they wanted to see you get married, is to arrange a live stream or video ….my nephew did that , I went round to his mum and dads a few days later and we had a mini celebration whilst watching the video…that was lovely to watch, his parents enjoyed sitting through it again and commenting on people’s expressions and reactions, which of course they didn’t take in on the day,
Talk with people you’re not inviting- a voice call, up front before invites are sent. Explain your reasoning. Talk with mum and MIL upfront too, tell MIL you and fiancée will/have already tlak to everyone so they know what’s happening and why. MIL is mostly likely worried she’ll be on receiving end of comments about how her dc and you have excluded this person or that. Tell her to refer anyone complaining or disappointed to you, it isn’t her responsibility to sort out pissed off people. Tell you know that it’s not ideal, but compromises need to be made (unless she wants to foot the bill! )