AITAH for telling my grandparents that it’s their fault I don’t want a relationship with them anymore?

I (21F) told my grandparents last weekend that it’s their fault I don’t want a relationship with them anymore. I am the youngest of three. My oldest sister (26F) has always been my grandparents’ favorite, while my other sister (23F) and I were always treated as less important.
This was very evident in certain situations. For example, once I had no way of getting home from school because the buses weren’t running and my parents were at work. I asked my grandparents weeks in advance if they could please pick me up that day. They agreed. I went to school without any trouble, but when it was time to go home, they called me to tell me that my sister had called with a similar request, so they could no longer pick me up. Mind you, she could have gotten home by herself, while I was stranded 25 kilometers from home. (To my sister’s defense, my parents knew about this situation; she didn’t know the buses weren’t running.)
Situations like this happened often to me and my other sister. A couple of years ago, on the recommendation of my psychologist, I invited my grandparents over for a conversation. I told them how I felt and gave them several examples of these situations. The whole time, they told me they understood and promised they would change. Coming back to the present, nothing has changed. They are still exactly the same, and I have given up trying to change it.
Trying any longer makes me unhappy, so I stopped. I get angry and frustrated every time I see them because of the resentment I hold toward them, but I remain polite, just distant. Apparently, they noticed and finally asked me last weekend why I act this way around them. I told them straight up that I don’t care about them anymore—that they proved to me that they don’t care about me, so I stopped caring about them. I reminded them that I had talked to them years ago about my struggles and that they changed nothing.
Now they are angry with me and told my mom that I need to apologize to them. My mom told them no, because she believes I am in the right. My sister, who now knows the full situation, told me that I am indeed right but should apologize just to keep the peace. I refuse.
So, AITAH?