This is the most complicated thing in life. As mentioned by others, you are still an outsider and you have not art of the family yet. However, there's still other was you could help them.
1. Talk to your gf, advise her to talk to the parents. Point out the weaknesses and stress it to her that it the brother will not be moving forward in the future.
2. Get her to talk to the parents and stay with her throughout the discussion period. Remember not to say a word until u are asked of your opinion abt the brother.
3. If they indicate that they want to listen your side of the story, tell it gently and progressively. Patience is important role. Starts with his behaviour, current situation and lastly the aftermath.
4. Respect the parents throughout the time.
5. If they scold you and saying you are outsider. Steer out from the conversation. Let it silent for a moment. Let them restart the talk. Keep drinking your water.
6. If they still can't accept. End the conversation. Give excuse u need to bring ur gf out to meet friends, meeting. Be very creative. U may want to walk out fast, but remember ur gf will still there to endure the stress. Most important is to support her.
7. If everything fails, ditch the gf. Many fish in the sea.!!! hehehe...
I had worst scenario before. My wife's sister (back then still gf) married to a libyan while studying in UIA without the parents knowledge and off course convert without them knowing too. This led to misconception of muslim by my parents and me as a muslim, feel ashamed on what the Libyan has done to the family. He didn't allowed the wife and the children to meet the parents bcos previously my parent in law converted to muslim and then renounce back to pagan (Happen in Datuk Harris time!! Sabahan knows the story as all policeman where told to convert or shall be sacked) and yet when they want to start a business, my wife's parent gave the money until their business lingkup, bungkus and move to Libya. I have talk to the family and advise them and it takes many many years to accept me and my religion. I went down to KL, scolded my SIL with the husband, took the children for picture taking and bring to see the parents. All this without my wife knowing.