Why are sibling relationships more difficult at maintaining the older the siblings get?

I have a sister. She’s two years younger than me. She has a car, I’m working toward getting one. I rarely ask my sister to take me anywhere. Whenever I have gotten a ride from her she offered but the few times I have asked without her offering she said “no” in her own way but rather than directly say no she gives a million reasons to why she can’t take me but that’s neither here or there because it’s her car and if she doesn’t want to take me that’s fine. Regardless, I still get done what I need to get done. Most recently, she again indirectly told me no again and I left it at that but deep inside I can see and feel the intentions behind it. She only wants to take me places if she offers to. My mom asked why I always get my groceries delivered rather than ask my sister to take me to the grocery store. I honestly told her, not in a messy or vindictive way but in a honest way, that my sister doesn’t really like taking me places. It was my truth. My sister reacted to what I said (which she had a right too) but she was upset, I wasn’t. It was like it triggered her that I actually spoke up against her. I’m mostly quiet, I never call her out, I’m mild mannered. She got use to that so the few times I do speak out about how I feel it makes her feel a way especially since I believe she knows it’s true but she denied that it was. She then started to throw in my face the few times she did take me places but once again it was only when she offered to, not when I asked meaning what I said still stood to be true. Before I knew it she was calling me “Weird” “Crazy” and told me I’m always being messy and starting things when the reality of it is I rarely speak up for myself. I’ve always been known for being quiet and chill. Then she said that I needed mental help. Even though I have no mental diagnosis. She then left the house upset and stayed the night at her friends house.

Why are sibling relationships more difficult at maintaining the older the siblings get?
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