Family Member Drama

So I try to think that I'm a relaxed bride. Now, I am indecisive and spend a lot of time before spending money on something, and I want everything to be right, but more on my end and dealing with vendors. In terms of the wedding party and families, we're really relaxed. We're going to DIY decorations and have them a bit mismatched, and the bridesmaids are going to pick their dresses (while staying with the same color and length and everything). I don't really want to delegate people on having to plan things, and instead just let them help where they want!



However, here comes in my fiance's aunt. We love her, and she's always been supportive. But lately has been flaky. She skipped my wedding dress shopping with almost no notice, she backed out on coming to his college graduation, and we couldn't even hand deliver the save the date because she went to home depot.... So it's been weird.
Well my fiance's sister is one of my bridesmaids, and the rest are my friends. Heres where I'll use letters to help: S - Fiance's Sister, A - Fiance's Aunt, C - Fiance's Cousin and Aunts Daughter. So S tells us she doesn't think it's fair that C isn't in the wedding and I should make her a bridesmaid because she feels excluded. Mind you, C is in 7th grade, and we're not really close. My fiance is mad that S even brought this up and tells her it's our wedding and if people are going to be more mad about not getting to be in the wedding than be happy for us, than that's their problem. S then says she'll just drop as a bridesmaid then because she doesn't want to hurt C's feelings.
Then today my fiance's mom calls him and tells him what A said. A said they just felt really left out by not being a part of the wedding, and that "Julie is just planning the whole thing and not letting Walker have a say". A said that C deserves to be in the wedding and shouldn't be left out. Maybe she can cut the grooms cake or brides cake so she has something to do.
Since then, my fiance and I have honestly just been mad and frustrated. We plan everything together, and a lot of stuff he doesn't have a strong opinion on so I make the final decision. But there hasn't been one thing we've done for the wedding that we haven't done together, since we are totally paying for it ourselves. Like it's really frustrating for family to be so entitled. We actually had wanted to ask A to help us with wedding flowers, but haven't seen her in over 3 months because she bails.
I mainly just needed to rant and get it out, but if you made it this far, I have two questions:1. Are we in the wrong? Like if I'm being a bit of a bridezilla, I want to know. 2. Is there any little job we could give the cousin? Like theres no way she's helping with cake cutting. We're not probably going to have programs or anything because we're trying to save money. And then if we asked her to help serve food or clean dishes, then we just feel like we're putting her to work which isn't fair. So is there anything that we could maybe include her in that I'm not thinking about? Like I don't want people to feel excluded, but it's also our wedding