Didn't you say that he took your dad's car?
Well, could he take it again, without asking? Yes.
Could the car end up in a swimming pool because he (The BF of your sister) was drunk while driving? Yes.
How would your dad react then? Don't tell me he would buy a new Mercedes and leave this one in the pool, without beating his ass up.
P.S. Do something to make him leave, without anybody knowing that it was you.