I'm going to embarrass my cousin for masturbating in my bed
What are you going to say to her?
I want to tell her "I know what you did last night".
You shouldn't shame someone for doing something completely natural and normal. While it was strange for her to do it in your bed, masturbation is a healthy part of being a human.
I guess you're right. People do weird things sometimes.
That they do. If you still need to talk to her about it just ask her why she did it in your bed. But be sure to let her know you arent trying to embarrass her or anything, just that you would prefer her not to masturbate in your bed of all places.