So, I have this head canon: Every year, John is invited to Mr and Mrs Holmes for Christmas, and it becomes a regular thing for both of them to go along. When Rosie is about 5/6 (and already calling Sherlock Uncle) she hears kids talking about their grandparents at school, and that Christmas she calls Mr and Mrs Holmes Grandma and Grandpa, and John gets all embarrassed and tries to apologise, but Mrs Holmes eyes light up and they respond immediately to Rosie calling them that and it becomes a running theme even when Rosie learns that they aren’t actually her grandparents. When John questions in private whether it’s actually okay, Mrs Holmes replies with “She’s as much our Granddaughter as you are Sherlock’s brother.” And Sherlock and John have been treating each other like family for years so he knows exactly what was meant by that.
Just imagine the conversations as Rosie grows up: “Grandma, can you help me with my homework please?” “Granddad, why does Uncle Mycroft carry an umbrella all the time?”
And when John needs some space, Rosie gets to go and stay with Grandma and Granddad, and she loves it because they always smother her with the best things. And when the hard conversation comes and John has to explain what happened to Rosie’s mother, the five of them (Sherlock included) are all sat around the table and talking about how her mother was a hero who saved Uncle Sherlock’s life.
