My mom was kind of like this, but not to the extent of some of these stories. She and her second cousin were both only children, and close, like sisters. My "aunt" as we called her, has 3 kids. My oldest cousin is about 4 years older than I am, the next is about my age, maybe a year older, and the last is 10 years younger than the oldest.
My middle cousin is selfish, rude etc. I wasn't that close to older cousin due to the difference in age. We didnt' see them all that often growing up, but my mom wanted us to be close, I think like she and my aunt had been.And she never really got it when I told her cousin was "mean" to me, as she was. She just tried to tell me to be nicer to her.
Mom and aunt kept in touch as we all got older, but I really never saw them. After both my aunt and uncle passed away, my oldest cousin came to visit my parents, and I was there too. She and I are now close, in fact, she tells me she thinks of ME as a sister! But more importantly, younger cousin is still selfish, etc. adn my mom finally gets it. I htink older cousin telling some stories too helped her realized what a witch with a b my other cousin really is!