Whether it's a holiday dinner, or just an impromptu gathering, chances are her family put a decent amount of time and effort into preparing for the event. After thanking them for the wonderful meal, rather than going off to watch TV with her brother, stay behind and ask if you can help clean up. Even if they refuse your help, make a point to clear the table.
This is the kind of stuff parents, particularly moms, remember. It's the tiny gestures of kindness that will make you stand out in her eyes.