Sherlock isn't Mummy's real son. She finds him very young, (about 5 years old), very sick, starved and abused, or even nearly killed. She makes sure he gets removed from bad people that have done it to the little boy, but she's too attached to him now and the boy clings to her all the time and won't let go, so she decides to adopt him and her husband accepts her decision.

Bonus (but it's not a necessary part): later in life Sherlock deletes everything before Mummy (or just prefers not to remember), but one day it all comes back to him, triggered by something when he's adult (maybe when he already knows John) and realises how deep the trauma is. Maybe Mummy's comfort is absolutely vital when he breaks down.