A Brother's Price by Wen Spencer
Other than that, it's a fairly straightforward historical romance, where the beautiful but poor son of farmers falls in love with a woman far above his station, etc. etc.
I wasn't too into it at first, and I really wished it had taken the role reversal thing to another level somehow. Other than the different family structure [which was cool--basically one man will marry a whole family of sisters, and they have like 30 kids in order to have at least one or two boys] you could have just switched all the pronouns and had a pretty normal romance novel.
Gradually, though, it grew on me and I found myself shirking my very important chores and so on to find out what happened.
There was a lot of interesting stuff about their society that was only touched on in passing, and it made me hope for a sequel that would really go into the issues that only got a surface treatment here.
Notes From the Labyrinth says it much better than I did, although I don't mind the "clair" thing like she does. (A useful term!)