I don't have any siblings, but I can say from an outside perspective of being around a lot of friends who do have older and younger siblings the relationship here really felt pretty normal as far as some siblings go. Not all, of course, but I don't think it's unusual for an older sibling to find a younger one annoying, or not wanting their friends to get along with their siblings. It's something most people mature out of though.
I've loved this one shot for some time, I feel like it captured a lot of things and reflected real life emotions and relationships very well and I love it for that. This author is by far one of my favorites, they're amazing at really grabbing you and shoving emotions down your throat.