You can sort this without calling the UN for support. Start with your dad’s aunt. Explain that you’re limited with numbers, inviting her means you’d have to leave off someone else, and that the venue is not easily accessible. If that’s not enough, say you’d really like Great Aunt Mable to be there but your in-laws are insisting on an extra guest and they’re not as reasonable and understanding as your dad. Sweeten the deal by visiting Auntie M before or after your wedding instead.
Pacify your mate by putting her fella on the reserve list, promising you’ll invite him if anyone drops out. Ask your brother’s reasons for insisting his kids come and help find a solution, while tactfully and firmly holding your ground. As for those people you really would like to invite but just can’t fit or afford, apologise and say you’re limited on numbers and imply that if it was entirely up to you, that they’d be there.
You can still have a fun wedding without losing your sanity, your best friend, or the will to live. Breathe deeply, and enjoy the excitement and satisfaction of having organised what will be the best party of your life!