IMO, you are all over reacting. "Wedding" is not a 4 letter word. If you are a decently versatile photographer then a wedding isn't that big a deal, especially a free one where the couple has been warned they may get what they paid for. Being not much of a party animal, I'd be just as happy doing the shooting as being a guest. That's mostly a problem when a new mommy buys a camera to photograph the new baby with no preexisting skill or ideas on what makes a good photograph. That's not the case here. Anyway, judging the photos comes after the fact and is the job of the newlyweds, not the photographer. My concern for bias would be that your attachment is to either the groom or the bride, but not both--in this case the groom. You might tend to provide great reception coverage of the groom but only get the bride when she is with the groom. The size of the issue depends on how much you like your new sister-in-law. If you think your brother caught a good one then it'll be easy to do well. If you think he ought not be marrying her then you'll really have to work hard to cover her equally.