I want to spend quality time with my younger cousins without caring for our intellectually disabled cousins
Why do you want to include some cousins and not others?
I will play with them, take them out for a pie, get to know their hobbies and get to know them better.
It's rude to only play with or spend time with the ones that aren't disabled. You should play with and spend time with all of them.
I really don't feel like sharing my time with those who can't keep up.
That is really hurtful, just because some of them might be disabled they still feel pain and rejection. Your other cousins will see how you treat their siblings and they will either think like you or reject you. You are an influence on them, so you should act like a good one.