First, you should remember the things which are not proper to bring, such as clock, medicine and so on. I think you should ask your girlfriend for help. Maybe she will give you some good advice.
If i were you, i will bring some american-featured presents. Whether the presents are cheap or not, the family members won't care.
In my mind, wine could be the best choices for her broters. You can bring yor girlfriend some cosmetics. As for her parents, you can bring some local products.
Wine could be a good choice for translator toO. Do not worry, everthing will be fine, and Chinese familys are quite good. Wish you can be part of her family. If you propose a marriage and succeed, remember to tell me :)

Thank you. Mr. Jim. I will probably bring a nice, smooth buttery/oakey Chardonnay for the brothers and the interpreter, and perhaps some some necklace/pendants for the sisters and possibly Mom. Looks like my checked bag will gurgle a bit! hee hee! I will tie red and gold ribbons around them before I leave!

Thanks again!

Hi, necklace could be a good present for the women, you are so clever. Hope you will ahve a good time! By the way, do not take too many in case of trouble at the customs.