Am I weird for hanging out with my sister and her friends as postgrad?

I'm 23 (girl) and recently postgrad living in the city I went to college. My little sister who is 20 still goes to school here. I'm close with her and friends with her roommates. Is it insane if I go to parties at her house with her roommates and friends? Is it weird to study outside the school library? I live a 5 minute walk from campus still in my college house and it's so convenient there but I feel sooo old and im afraid of people seeing me and thinking im a freak who is still trying to hang around college students