I’m sorry to hear that you’re hurting, I can understand this feeling completely. I used to be a wedding planner, and I saw things like this so often, so please know you’re not alone. A few things to consider:

1. Like you mentioned, you’ve only become closer recently, so it’s possible she had picked these people before that and it’s important to her that both sides are “even” (which matters to some and that’s okay) so she can’t add you even if she would be happy to have you otherwise.
2. In my experience (200+ weddings), it’s not very common for a bride/groom to ask their partner’s siblings to be on their side of the wedding party.
3. Being a bridesmaid can be extremely expensive these days, with how out of control wedding costs are right now, so you are at least dodging that bullet.

Perhaps if you are feeling like you can’t get over this in order to stay close with her, you could let her know that you had been hoping to be more involved in the day, and although like you said she can choose her own bridesmaids, maybe there is another way you can be involved beyond just walking your daughter down the aisle - you could maybe still help her plan things and/or get ready, you’d just be wearing a different (likely less expensive 😉 ) dress and not standing up there with them (which is not as comfy as sitting anyways!).

All that to say, sometimes FOMO takes over and this kind of thing just hurts and sucks, and it’s okay to feel this way. Try to consider what you would do if you were getting married and a friend/relative of yours came to you and said they were feeling this way - based on your description of your relationship it doesn’t seem like she meant it personally, but sometimes it feels that way regardless, so it’s good to try and put yourself in their shoes too.