My husband's relatives have described me in this way. I have been told that in not making my rounds in the party conversations and dancing with everyone else, I am looking down on them. My reason--I am more reserved and a little awkward socially. I am not comfortable dancing in small groups, and small talk is very difficult for me. I go around and speak to everyone upon my arrival, and then I find a seat where I can enjoy everyone else enjoying the party. I smile and truly enjoy myself. I make it a point to show up, but I have been accused of only coming to please my husband and show off my good looks. When I can't show up, I feel an obligation to explain why, but I get accused of thinking that my life is way more important. I have not shown up to an evening birthday party because I was tired, and I felt terrible giving that reason. What was behind my tiredness was the fact that I was a newlywed second wife dealing with that adjustment and the in-home caregiver for my terminally ill relative. A few months after her death (which was absolutely devastating to me), I could add to my list a move to the outskirts of town which significantly affected my commute to ANYWHERE. When I did show up, sometimes it was just to have something else to do. I used it as an excuse to wear make-up and high heels, and they accused me of dressing up to show that I was too good for them. (I wasn't in a prom dress, just jeans and heels and make-up.)
These few years later, you know who I have relationships with? The ones who just accepted me for me and didn't criticize me for not being like them. The ones who sat next to me and talked to me and listened, getting to know who I am. Today, we make plans to spend time together.
Maybe you turned this person off or offended her in some way, and her response was just to back off instead of confront you. Maybe you're just not her cup of tea, and she recognizes that it's okay not to feel obligated to show up for you. Maybe she comes from a different family dynamic and doesn't place value where you place value. My SIL and I do not get along. She doesn't understand how I can not push our children together. Well, my son has my family to spend time with. Also, there are issues with my stepsons that concern me, so I she and her issues are not even on my radar. I am focused on what is happening in my home and my life and not on how to relate with someone I don't have to relate with. I used to get really hurt when my husband's relatives wouldn't RSVP. They would view invitations and just either show up or not, giving no indication of whether they'd even received it if they chose not to attend. I always made sure to respond to their invitations, even to decline, and they kinda brushed it off because that's no big deal to them. (Of course, that made me look like an uppity social butterfly, because I always RSVP.)
Anyway, my point is that this sounds like it's just a personality difference. Those are okay. Don't hold it against her that she doesn't respond in the way that you think that you would in her shoes. She is busy living her life. Next time you see her, if you are sincerely interested, go and ask her how she's doing, and then open the door to really listen.