Different people grieve in different ways. The time they take varies dramatically, I don't think it is fair or reasonable to tell another person how they should deal with the death of a partner, IMHO whatever gets you through the ordeal.
I do think men and women behave in different ways, and perhaps in general men are more likely to find another partner quicker (although my Fiance worked with a girl that was dating another guy within a week of her boyfriend being killed in a RTA, she even asked for time off work to go to the funeral, and promptly took off with her new boyfriend for the weekend). Some of that may be down to a desire for sex, although I think intimacy and comfort is probably a greater driving force. But I think a lot more is because men usually have much less of a support network of friends and relatives about them than women do. It is very likely that most men in that situation would find it difficult to turn to their friends, colleagues and relatives for comfort in a difficult time like the death of a partner. It is much more socially acceptable for a woman to ask for ( and receive) comfort and understanding from her social group than for a man to do so.
In this situation, maybe he did know this person before his wife died, that doesn't mean he was "involved" romantically with her before the death, but that she was an obvious person for him to turn to. Maybe he was already having an affair with her, maybe you're looking for someone to take out your hurt on and your BIL and his new wife are the easiest for you to do it with. All are possible, but I tend to agree with Carol Anne that the feelings you are showing are likely to do you more harm than good and that you need to give a great deal of thought to your actions and how they effect your nephews. It wouldn't be suprising if they are taking their hurt out on the person they see as replacing their mom, therefore however well she behaved, she may get critisism from them.
The only thing worse than losing a family member is having something like this happen afterwards.
Your BIL may well feel exactly that way, he's lost his wife and he's very likely aware that he's getting attitude and critisism from her family.