I don't know if this is really a spoiler, but just in case, posting here rather than the other thread.
http://ew.com/tv/2016/12/28/sherlock-season-4-darker/

While much has been speculated about the impact of Watson’s marriage to Mary and the arrival of their newborn, the addition to the family doesn’t disrupt their relationship in a major way. “As in the original stories, Sherlock has no problem at all with Waston being married,” he points out. “Sometimes that’s portrayed as a kind of jealousy thing, which is a bit weird.”

So no jealousy.  Not that that's a clincher by any means - he might just not be a jealous person!  But it could be another clue.