How do i sneak out of a room?

so i am staying with my little cousin right now and i want to have fun. we have to sleep in her moms room and we want to have fun. she says when her mom is asleep she sneaks out of the room and goes to the kitchen to eats some food . so today she build this really cool clubhouse and we both want to sleep in it. i know this is a little bit weird cuz i am sixteen but this is so fun. her mom wont let us sleep in it but ohhhh we will how do we sneak out without her knowing also she has a night shift at a taco cabanna so sometimes we can but sometimes plllleaeee help