I'm a proud bibro (and not in the slur sense some people tend to use it) and I think that Dean choose to trust Sam and his judgment, for the time being. Why would it be so ooc? Plus, Dean want to protect his mom. There will be tension because the bmol is not clean, but I have faith in the brothers.
To me personally, as I see it, the BMoL have never done any real personal wrong to Dean. They did (or at least their rogue did) real personal wrong to SAM. So Dean, himself, only has ‘you hurt my brother/my brother doesn’t like you’ to hold against them. He has no other reason to dislike them.
In that same vein, Dean now also knows the person who hurt Sam was a rogue and not the BMoL themselves. So with that part of his ‘hate’ out of the way, the only other thing keeping Dean at bay from joining would be Sam’s dislike of them because SAM was the one wronged not Dean (and out of respect, Dean wouldn’t join if Sam wasn’t willing). Thus if SAM is willing to work with them the same way they work with PLENTY other bad/questionable guys, then Dean has no reason to not agree and go along with it. They have come a long way and Dean trusts his brother and will stick by him. Two heads are better than one. Dean’s presence keeps Sam safe, keeps his mom safe, just by being there.
He might not agree, and neither he nor Sam trust the BMoL, but there’s no denying that they ARE doing what they said they were gonna do. Ridding the world of monsters one step at a time. Sam and Dean would be OOC to just ignore that and disregard it, when they could learn so much FROM the BMoL when it comes to innovative hunting tech and gear and methods.
The clash is going to come when the boys realize the BMoL also kill ‘good’ monsters, and as far as I’m concerned, I look forward to seeing how that plays out. Be it either the boys quitting, or teaching the BMoL a better way, OR simply taking them down from the inside.
Do I wish Sam had told Dean sooner? Yes. Do I think Dean didn’t have a clue? No. Dean’s not stupid, he’s hella perceptive and good at playing dumb when need be, he pretty much knew what was going down and wasn’t surprised. He was just waiting for Sam to confess so they could be on the same page. They’ve been through too much and have matured from the ‘hold things against my brother’ ways of the past that has always led them to ruin, there’s no reason to not stand united and be on the same page. No reason to not trust each other, and trust (and protect via joining) their mother. Family 1st.
Either way, Sam and Dean will have each other’s backs and will rip the BMoL if it comes to that and I’m really enjoying this storyline.