I don't think there is anything wrong with it. It all depends on what kind of friendship the brothers have. When I was in my early 20s, there were two instances where my sister and I both dated the same guy at the same time. And in times past, a man would often marry his brother's widow or move in and take care of her if she was abandoned. This happened in my family, and I've often wondered if there were any liaisons that happened while my gggf was still alive because they did all live together (we had some other strange configurations as well, like when my ggm married her half-brother's son, her nephew). I am sure there were more than a few times when brothers did share one wife, but those don't get written in the history books!
Your husband's brother may not be hep to the idea, but if he is, I think your inclination to communicate, communicate, communicate is spot on. You would do that no matter what, anyway. I don't really see a brother as that much different as any other additional partner, but the key thing to look out for would be their family dynamic and whether it is healthy, competitive, weird, or whatever.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein