Because you will be staying at his family's house, you will be in the hub of activity with a festive atmosphere and a perfect time to get acquainted with his family prior to the wedding day. There will be so much going on, which is good, because the total focus will not be on you, it will be the wedding. A great time to get to know family, for sure!
Like any good guest, you should bring a hostess gift. A good and welcome hostess gift is one that can be consumed. So, perhaps you can begin to gather things for a nice gourmet gift bag. Go to Home Goods or TJMaxx or Marshall's and pick up some nuts, fruit spread, gourmet crackers, muffin mix and that sort of thing. Food is a universal language and a great gift. If you want to give something they can keep, make it small, like a lovely knife to spread jam or a cute honey dispenser..something small.
Because you are the girlfriend, you will be considered as family and I am sure they will want you to sit in close proximity to them at the ceremony. As far as the reception is concerned, it will be one big party where you will meet so many people your head will spin. So, your job is to smile and stand up straight. Easy peasy.
I actually think this wedding and the environment surrounding the festivities is a great time for you to meet everyone. There will be a great balance of your time with them and diversions related to the wedding.
As a house guest, offer to help set the table, and be on dish detail. Maybe they will tell you to go sit down and relax, but offering to help shows you are a team player. Make your bed, and do the typical things you normally do to keep things tidy. Keep your bathroom toiletries in your bedroom.