It is very clear she is nervous of this wedding because of the way she has behaved in the past at events with Brian's family. If Keith gets any form of attention, even from a female cousin she will not be able to help herself, she will get pouty and nasty to him and make is so obvious she is jealous, then make a spectacle of both of them, I have seen her do this many times!
So she will go to the room on her own? That may be the intention today but once she gets in one of her drunken tempers there is no way she will sit upstairs on her own and let Keith enjoy himself without her, her only saving grace is that Keith will never get near as much female attention as Brian did, poor Keith would probably only get glances of pity when they see his teeth while Brian will always get girls checking him out, poor piggy Sarah was so jealous!
