I don't know how specific this is to my family, I assume it happens with other people, too, but I'm not sure how frequently.
A little backgroun information:
The sport of choice is hockey, basically the only sport that the majority of my dad's side of the family cares about. There are 3 teams; Montreal Canadiens, Ottawa Senators, Toronto Maple Leafs. The Canadiens have the most support, having my grandmother, one brother, 2 uncles, an aunt, and 2 cousins. The Senators have my dad and grandfather. The Leafs have me. My mom, one aunt, and youngest brother are neutral here, and pretty much stay out of it. So, basically, any family gathering with anyone on that side of the family inevitably turns into a discussion on how the Leafs haven't won anything in 40 years. Doesn't matter the time of year, doesn't matter the standings if it's during the season.
The only person I have on my side, even remotely, is my grandmother. Apparently, she's mad enough at my grandpa from jumping ship from Montreal to Ottawa a few years ago that the Leafs are her number 2 team, something my uncle can't seem to understand. I understand that the Habs and Sens don't have the playoff history and rivalry that the Leafs and Sens do, but it's still interesting to me that they seem to be willing to overlook the fact that they still are division rivals to gang up on one team (this seems to happen most places, too, not just my family, I've noticed).
For example, the other day I was at my parents' house, doing some laundry, and the Leafs/Bruins game was on TV. My brother (Habs fan) and dad (Sens fan) were both watching the game with me, and actively cheering against the Leafs. Never mind that, for bother Montreal and Ottawa, it was probably better that Toronto one in terms of the standings. I realize that part of it is showmanship on their part, just trying to get under my skin, but I still find it interesting, considering that Boston is a division rival of both of their teams, too (and, Boston and Montreal are probably two of the biggest historical rivals in the NHL).
I guess I'm just curious as to how common it is to have so many different favourites who happen to be rivals within one family. I realize that a lot of it has to do with geography (Ottawa's an hour away, Montreal 2, Toronto 3-4) and if we lived in some place with less choice there'd be less favourites, but I've always wondered what it would be like to live in a house where more than one person are cheering for the same team. About the only time you're going to get us on the same page is when there's an international tournament going on.