I don’t want to go to my cousins birthday party because I simply don’t want to and i don’t have the energy to

I don’t think it’s rude considering his birthday was a while ago and it’s far away I just came back from a trip and my sister left that trip and couple days earlier and she’s always trying to find something to be mad at me for so now she’s giving me an attitude and trying to guilt trip in in to feeling bad about something that doesn’t matter she wasn’t angry or disappointed that some family members flaked out on my graduation and I didn’t really care I don’t get it no body else cares why is she always trying to find something to be mad at it’s not even her birthday I don’t get family sometimes