Fiance's cousin wants an invite to my bachelorette

I'm having a bachelorette party that my sister is throwing in a few months and my close friends are coming to my party.

My fiance has a large extended family. His younger brothers girlfriend hangs out with the family frequently, and then there are his older cousins. They are interested in coming, but I don't know them that well, I don't feel like I can be myself around them fully and didn't want to invite.

When I got engaged, his older cousin who is in her mid thirties came up to me and told me let me know if you need help planning your bachelorette. I thought that's nice and all, but I didn't know her very well. I haven't spent time with her much since I've met her and we are very formal with each other. When she said that, I felt odd because obviously my close friends and sister would plan it.

Every time I have met her in the last year, she asks my fiance or me about my party. My fiance and I avoided the question because we are under a lot of stress for the wedding and said when we decided fully we would tell people. As a result of this, we are both thinking we should do a separate party with extended family - have a joint family bachelor/bachelorette. Otherwise, it may be too many people and you're running around trying to talk to everyone.

I think she automatically assumed that she isn't invited to mine and turned from a sweet person to a mean girl. She now is ignoring me in parties and even got her husband to do the same to me and my parents. I can't think of anything I could have done to offend her.

We want to have the joint family bachelor and bachelorette party still, but to be honest I don't even want her there by how rude she thinks she can be to me. I also don't think she would be grateful for the family bachelorette invite either. I can tell by the way she talks to me she keeps trying to fish an invite along with my friends but doesn't even know them. Any advice is appreciated!