"your in his with me" the correct saying is "you're in this with me" You're= you are this saying means "we are in this together" you can use this saying whenever you do not want to do something alone or by yourself. You can use it jokingly ( as a joke) or seriously with friends or family members

Maybe you mean "you're in *this* with me" (a variation of "we're in this together")? It means that you are both involved in the same situation, so you are equally responsible for the consequences. For example, you and your brother agree to buy a car together and now that you've driven the car home, he changes his mind. You could say: "I thought you were in this with me? If you don't want to share in the cost, then I can't afford to keep it."