How do I deal with this difficult group member?

I’m president of an organization that’s been in existence since 2019. There are seven other members of our group. One of them is a girl who has proven difficult to work with. Whenever I message her to ask a favor of her (relating to our organization) she takes a week or more before she responds to me.

Sometimes I have to message her brother to get her before she responds to me. I’ve felt bad vibes and energy from her towards me for a long time. My brother is also a part of our executive team and she is much friendlier to him and all the other guys in the group. But she’s very indifferent towards me. I also notice she never likes my pics on Instagram (but likes everyone elses) Not to sound superficial but it’s something that I’ve noticed

Whenever I pledge an idea she never shows any excitement or enthusiasm. Even when the other members do. When I schedule meetings she misses a majority of them and doesn’t bother noting she won’t be able to make it, even when she sees our messages. She pops up into the meetings whenever she feels like it and starts suggesting numerous ideas and initiatives and yet is rarely ever involved in our previous meetings. She’s shown up a little.more recently because our annual convention is coming up and youth from all over the world will be attending.

I’m starting to suspect she’s jealous of me. I did research on her and found out she used to run an organization in her school before we started this one.

She has made it known that she will be in attendance. I personally don’t want to work with her because she is defiant, unpredictable and stubborn. I recently messaged her on our group chat to ask about a task she had volunteered to create for our group and she read the message and hasn’t responded.What do I do with her? How do I handle her without seeming salty or aggressive? Especially at this upcoming convention.

Also I do not want to directly address her because she knows what she’s doing and I feel it would be a waste of time. Also I can’t trust that any of my executives would back me up because they’re nonconfrontational, passive or don’t care about that matter.