It all depends on how much you like the girl and how long you want to be with her.
If you really like her, and want to have a... SERIOUS RELATIONSHIP... then follow the good advice given and don't fuck up.
If you just like her enough to get into her pants, then maybe you shouldn't try so hard. You can crack some jokes at the little brother's expense and get away with it. Don't insult the parents, but you don't have to wow them either.
If you plan on dumping her ASAP, then... well do as you please. You can break the little twerps arm and make fun of him for it! Fart as you please and piss in their closet. Hell, you can try to get a threesome going with the mom and your gf if you really don't care. To top it off, I would start setting things on fire as I left.
So now you have three options for the three levels of relationships!
Your welcome.