I'm mad that my girlfriend wants to spend entire weekends at her family's house
You shouldn't be mad she wants to spend time with her family, what's wrong with that?
I understand where she's coming from but it would be nice to spend time with her too, once in a while
You shouldn't be bothered if your girlfriend spends weekends with her family. If you want to spend more time with her, you should ask if you can join her.
What? That's how you win a lady over.
Yes, honesty is the best way. I am sure she will be flatter and want to spend more time with you.