Obviously their dynamics will be what they well be, but... I really like the model of thinking of metamours in co-primary situations as being like siblings. Siblings have, ultimately, one thing that binds them together -- their shared relationship with their parents. Similarly, metamour co-primaries are bound by a shared relationship. Siblings may be similar or different, they may be extremely close or they may fight, but at the end of the day they know that their shared relationship will pull them back together again and again, so they need to at least be cool with each other and, ideally, should be allies.
Your BiL's reaction makes me think of when my roommate, Eddie, came out as FtM trans. Everyone kept asking "Oh, what about poor Liam?" (Liam being Eddie's husband). Now, a couple years later, no one asks that anymore because he's obviously fine. But people just couldn't believe it at first.
What a shame that people can't mind their own business.
I hope Sward feels better soon!!