before the patch I noticed that it was always the oldest heir that became king and all his brothers would then be promoted to Royal Blood (this became a right pain because you often got an unmarried 58 year old taking over and when he dies civil war erupts)

Post patch, I have noticed that if the heir to the throne is too old the AI now selects one of the younger brothers to be the next in line (hence solving the problem on the old monarch who couldn't get it up and make any kids).

Just keep those brides rolling in and you'll do fine.