Codes – riddles, really, a form of amusement for a Holmes and bewilderment for many others. Most people you simply solved the traits of, acted accordingly and had them in your hands. John wasn't like that.
He had puzzled both Holmes brothers from day one, leaving Sherlock fascinated and Mycroft wary as his governmental position had taught him. Somehow the man had integrated himself into Sherlock's life – and Mycroft's, by extension.
Contacting John's next-of-kin in this situation was hardly required, and helping her escape rehab was certainly beyond what was called for. Mycroft briefly wondered why he hadn't just called Lestrade or someone else to care for the man, why he felt obligated to care.
After all, John was not his brother.