Heart is in two

Well obviously nobody here can tell you whether or not this person is actually making flirtatious gestures towards your girlfriend, however, I would say that it is somewhat likely (in my opinion anyway). If it is the case that he is actually doing these things intentionally, then what I would recommend is that you keep your distance from him a little bit. If you think he is trying to come between you and your partner, then he has to be stopped in his tracks. Perhaps try to avoid contact between him and your partner if that is at all possible.

You may also wish to consider talking to your partner about this. If you are in love, then you should be able to explain your concerns to her. Just tell her that you think he is trying to come between the two of you and that you find it very disrespectful and wrong. If I were in your position, I would not say I feel threatened (that could be very insulting for your partner). Tell her that his actions have negative implications and that you would prefer to stay away from him for a little while until he matures/ changes his attitude.

Of course there is always the option of talking to your brother, but that could easily end in further conflict. If you are wrong, he could feel insulted and if you are right, he could become competitive, etc. It would be best for you to talk to your partner about this and simply keep away from him for a while if you are overly concerned about this issue. I hope things work out for you :).