Funny how you failed to notice that at the start of this episode when Sam finds them the case, Dean’s all solemn and not wanting to go on smaller cases, and Sam persuades him by saying “we’ll get him back” referring to Cas who’s possessed by Lucifer atm (cont) https://twitter.com/Lattize77/status/1001723570588999681

Very much implying that Dean doesn’t want to lose his focus on getting Cas back- which has also been mentioned the previous two episodes.

OF COURSE he’s devastated by losing Sam, you would be if your brother who you’re close with died, but the watching the man you love die thing is just not relevant here in relation to Sam... on top of that, Dean didn’t watch him die- he did, however, later watch CAS die

Which broke him thoroughly, showing how he can’t deal with this new normal and shit. This is in no way a hint at wincest going canon, it’s not something that’s related to Sam and Dean’s current situation at all, just... no.