How do i get my gf to be more open when she gets back?

If you want you could get her flowers, and say welcome home when she gets back. Your girlfriends mom is right. This is normal. Unfortunately there is not much you can do about it other than be supportive and try and understand the situation. She grows attached to her family and then misses them when she has to leave. Its not that she doesn't love you, its just she misses her family. Everytime, she visits one side of her family she goes through the same process again. She starts out with not wanting to be there and then begins to feel like she is at home once she settles in. Then when she has to leave she gets upset. There is not much you can do other than learn to accept this.