Practically, Tiger, you are going to have to wait it out. You have identified your DPs state of readiness, your kids will take longer, but you will probably have to put up and cause the rift BIL is accusing you of, or shut up(ish) and wait for the natural to occur.
DH has an equally toxic set of sibs, I put up with all sorts of shit for many years. I was happy to be cut off from all that lovely family contact and let DH get on with it. It took a friend to call SIL on one of her really nasty little tricks for DH to fully appreciate what was going on. SIL used to give him a hug hello and, over his shoulder, smirk and mouth to me "I could have him, you don't deserve him, he doesn't actually love you". I had always laughed (DH dislikes her intensely) but a mutual friend called her on her behaviour in a packed pub. Much sulking ensued.
DH eventually came to the decision that he would see them occasionally but would never ask me to accompany him. Or to ever invite them to us.
Life has been easier since. He sees their behaviour for what it is, If they have a pop at me when he is with them he used to say I was working (teacher, so that was easy). Now he says I just don't want to visit. SIL had a conniption the first time, but he pointed out that we have lived here for 10 years and the closest they have been is the pub on the river.
Put your foot down gently. Give your DP the time he needs to see BILs behaviour as an adult, not as a spoiled child. But be prepared for a long wait. Grow a sense of humour about it (it can be done with practice, I managed eventually).
Good luck.