My situation wasn't all that different. My parents had more kids than they could handle, and as the oldest I got the least of everything -- and was required to give up my high school years to act as an unpaid babysitter and tutor for the younger ones. My mother wanted me NOT to go to college because it deprived her of her help; other issues were involved there too. I know they didn't have money, but I also didn't get any emotional support. In contrast, once a couple of us were out of the house and my parents had more time (and money), the younger ones got much better treatment.
Things between me and my mother are much better now, but I can't say I've forgotten all that. Regardless, I've never held any of it against my siblings. They had nothing to do with making that situation, and if I'd been younger instead of older I would've accepted the help given to me just as they did.
Realize, too, that whatever she does with her wedding is hers. Your choices have to do with whether to attend. Don't feel responsible for more than is actually "yours". I think that's an oldest child tendency.
You're right to try to treat your kids equally. It won't work out perfectly; they'll have different opportunities, etc. as time goes by. Everything seems to work out for my oldest: She was always assigned to the best teachers, fell in with good friends, was offered the chance for great extra stuff at school. My poor youngest often was assigned to lesser teachers, her peer group at school just isn't as "enriched" -- in fact, she has a bunch of bullies in her grade -- and things just seem to "disappear" when she ages into them. Nothing I can do about those things, but it has affected their upbringing.
