I can't think of one good reason they would notify Harry in advance of the announcement, but I can come up with plenty of reasons why they would allow him find out at the same time as the rest of the world. It's the only positive thing to come out of this, that Harry, and the world, knows for sure how irrelevant he is to his brother and his family.
Harry is paying the price for his attacks on his family, and his silence when their friends (Scobie et al) threw a barrage of insults and unfounded attacks at his brother and his wife. William will be doing what Harry said he was doing 'protecting his family' and keeping the viper away from them. I would be shocked if they ever have a meaningful relationship again.
I suspect William and Catherine's kids will grow up with a huge resentment against Harry too, when they realise what he personally did to their parents.