Just a thought but maybe after your (and your DP's) refusal to "bite" at the shock of the news, your MIL told your BIL about breaking the news to you and DP about their marriage? Maybe the BIL feels awkward and is delaying the inevitable?

Coming from a family where there are major issues between DP and his brother, it sort of changed the dynamics after we had DS in January. It was like they had this opportunity to mend things without really making a massive effort...sorry i am having trouble explaining it right..it was like they reached out to each other without realising it...the feeling was palpable while I was in hospital.. I hope this happens for your DP and his brother... the realisation that "family" takes on a whole new meaning let us know how it all goes.