In reply to mypyrex:
I'm very sorry to hear of your situation - it sounds very similar to my Grandfather. He lost his first wife when my Mum was about 4 and re-married. His second wife died about 10 years ago and he has never really recovered from that, and the experience also seems to have made all the grief from losing his first wife re-surface.
Like your BIL, he has become increasingly depressed and reclusive, refuses to join in social activities (he used to be a real social butterfly) or engage in any hobbies and rarely leaves the house now. He descended quite deeply into drink for a while and flirted with the edges of OCD. He has also turned his living room into a shrine to his two wives, which most of our family are finding kind of creepy (although I recognise that people mourn in their own way). The kinds of things he says, like "I should have gone first" also sound similar.
We tried similar things, trying to engage him with activities he used to find interesting, but without any real success.
He refused to see a doctor for a long time, and then went to please my Mum but refused active treatment for even longer. Last year, he contracted pneumonia and spent a couple of weeks in hospital, during which time they managed to dry him out (he hasn't had a drink since), did some CBT with him and persuaded him to start taking anti-depressants. They also got him some "home help" - a very nice lady who doesn't so much help as go and have a chat with him every day and pretends to be interested in his WWII memorabilia
He's not back to his old self by any stretch of the imagination, but he's a lot better and more manageable than he was.
I think the lesson there is that you do need professional help (although I don't recommend pneumonia as the method to get it!) - but also recognise that it might be a bit of a fight to get him to accept it - it took us about 5 years to get him actively engaged in treatment. Also, you aren't alone with this!
Regarding the point about counselling "making it worse" - this may be a short term thing. My partner is a widower and was seeing a bereavement counsellor for a while. He said that the process made him experience the grief all over again, but to engage with it more actively than he had the first time round. That was a hard couple of months, but it was worth while, because although it appeared to make things worse, once he came out the other side he was dealing with things a lot better. Of course, your mileage may vary.