Irene Adler turns up at 221B Baker Street with Sherlock's baby, the result of some fun and games the two of them indulged in after Sherlock rescued her at the end of "A Scandal in Belgravia." (It wasn't love, he was just passing the time while waiting for his coat to get back from the only good dry cleaners in Karachi, Pakistan). The baby is, of course, named Hamish Holmes. (And Sherlock's "safe word?" "Oopsy!")
Adler meets and falls madly in love with John's philandering alcoholic sister Harry. (Irene cures her drinking problem by smacking her with a whip and yelling "drop it!" whenever Harry picks up a drink.) They wed and raise the baby together.