@aog17 My mom and aunt are 10 years apart. Their relationship is different, but I wouldn't say it was bad. Things they didn't like:
Aunt - she hated that rules for her were different for my mom because of age differences, and still holds a bit of a grudge for missing out on things because one of her chores was being babysitter from time to time.
My mom - thinks my aunt got their parents in their prime while she got a more tired and strict version, she also holds resentment because my aunt got more time with their Dad who passed away rather young. She also hated her sister bossing her around.
I have 3 kids myself, and there's 9 years between my oldest and youngest. Their relationship is how you'd expect, the older has very little interest in the youngest and the youngest wants all their things which causes a lot of upset. You know typical sibling stuff.
TLDR: Not selfish, just prepare for even more of the joys of sibling life.