I had a massive falling out with a friend once, and I wanted to never see him again, and the MN consensus at the time was to cut him out of my life, he wasn't a friend, etc. etc.,
The tihng is, he was also a really good friend of my DP, and while DP completely understood why I was upset and was very cross with my friend for what he did, he didn't want to loose his friend just because I fell out with him. At first I was hurt, the emotional part of me wanted to tell DP he couldn't ever see the friend again and that if he loved me he would cut the firend out, but the rational side of me knew that this friend had done nothing wrong to DP and I had no right to dictate to him who his friend were.
Anyway, loing story short, for the sake of my DP, I met up with the friend and we discussed everything, he appologised, I appologised, we had a good cry about it all, and now I am friends with him again (it took two years to get to that point), and I only did it for the sake of DP and not wanting him to have to feel bad about seeing the friend because he knew I would be upset, make social situations difficult, etc etc and actually, I am glad I put it all behind me and we sorted everything out, because now I can happily call him a friend again.
I'm not saying you need to make up with your BIL, but you might need to put it behind you for the sake of your DP, if you love him you won't want him to suffer by not seeing his brother or feeling too scared to talk to you or tell you the truth.
You brought the lieing on yourself, he didn't tell you because he didn't want to upset you. It was wrong of him but you need to be more understanding and talk to him without getting cross, appologise for putting him in that situation, say you understand he wants to see his brother and he needen't hide it from you because you will not make him feel guilty about seeing his brother. You will then be a better stronger person for having done that, your DP will respect you more, and you will have less stress and upset in your life.