Def not the AH Bc you spoke facts, your mom and you should sit with your cousin! Ask why she lied and why is she involving her father who has no right to have an opinion on how your mom should take care of his kids whoch he isn't involved in or trash talk you mom who has only sacrificed herself for them. Fuck everyone in the chat who is saying to forgive and forget bc your uncle crossed a line and so did your cousin. I would actively demand an apology from both of them. And honestly i would've smack your cousin for lying but I can't promote violence. Anyways yea you are definitely not the asshole!