For years my nephew (now in his early 20's) would come over with his friends to play games and stuff and while they were there I would always nurse my babies as I normally would. It would take a couple of visits before his friends would get use to it. But now all of them, down to the last one, are very much lactivist and find it weird that they ever were uncomfortable around it!
I wonder if your BIL saw a good model of breastfeeding normalcy at some point in his life, or if he realized the truth through some other means? It would be interesting to find out.