Can cousins be the financial sponsor?

I'm starting the process for a student visa and would like to go in April 2026. However, I still have questions about the financial sponsor. Can cousins be financial sponsors? To prove this, I'd need his birth certificate, my aunt's birth certificate, and my mother's birth certificate, correct? Would immigration accept it that way?
It's very confusing because I've heard of cases where non-relatives were used as financial sponsors. And in a recent case, a great-uncle was used and the certificate was issued.