My girlfriend's cousin sleeps with her every day. (My girlfriend 2 1, her nephew 1 1) I'm bored to death. ...

LZ is too stingy! You can't be angry unless you think her nephew is having sex. If not, you think too much. Maybe your girlfriend was afraid that you would sleep with her, so she asked her nephew to sleep with her. Where else would you let her nephew sleep?

And LZ should be careful in case the nephew is sent by your girlfriend's parents to supervise how you treat her daughter! Anyway, you are jealous of a child, so be careful!