Hmm... I have a lot of half and step-siblings... Some of my cousins are my nieces and nephews and some of my uncles are my cousins... It's complicated...
I'm content with just forgetting most of them half the time, till they come over and I have to dig through my mansion(Mind) to remember who they are.
And their relationship with me vs how I should treat them. For example, my nephew who is my cousin. He is technically my nephew but I should treat him as my cousin because his mom is my mom's (the one I live with) sister. His father however is my cousin because his mom is my biological dad's sister.