Whatever you do, don't react to this unpleasantness. It may well be that they are simply trying to wind you up - in which case if you say or do anything they will know they have succeeded.
There is a huge distance between what is said and what is acted upon - these two are chatting about your girlfriend in an unsavoury way but that does not mean that they would actually follow their nastiness through with action.
My advice is keep your cool, stay away from them as much as is possible, and protect your girlfriend by explaining the situation gently so as not to alarm her unduly and persuade her to stay away from your father and brothers. Show the world that you are a man and in control of the situation - fighting will simply underline your immaturity and you and your girlfriend will be the ultimate losers.
Of course your mother won't hear bad things of your brother, she is programmed to protect and defend her offspring, so don't waste your breath complaining to her. Just keep a cool head and keep a distance between your father, brother and yourself as much as you are able.
So far, well done, you are talking about it and looking for advice, that is mature of you and you will find help. Good luck.