'You can not have dinner with Liz! she's your sister's ex mother in law! That would be like, sleeping with the enemy'
What does sleeping with the enemy mean in this sentence? and is it an expression? is it common?
Yes - very common. It means 'giving solace/succour/support/enjoyment to someone you're not supposed to be friendly with'. (It derives from wartime I think - occupied lands, young women whose menfolk are away, enemy soldiers....)