Yeh don't tell your girlfriend, it will upset her, and make her feel uncomfortable coming round yours.
Like say maybe you're brother has over exagurated the situation, maybe they just said in a nice way that the thought she was good looking. Maybe your brother just said wouldn't mind bit of that-in a jokey way rather than I want to **** her.
Your younger broter may have just taken it out of context.
Best thing to do is sit down with them and say what you feel, without killing anyone!
If you feel really aggressive then as you're about to go out, tell your father and brother as you are about to leave one day.
Just say clamly that you've heard what saying and not pleased. Then camly leave and go out.
This will give them time to mull over what you have said and for you to remain clam.
Then perhaps speak to them again about it.
If they value your feelings they will sit you down and expalin the situation, and presume apologise.
Good luck, just remember everyone makes mistakes and they may have just been having a joke.