Sherlock's sister

What if Mycroft wasn't the oldest Holmes child ? Desdemona is the oldest of the Holmes siblings. Desdemona being​ the oldest wasn't always around when Sherlock was growing up but when she was Sherlock and Mycroft were different people. Desdemona went to work with the military so she didn't see Sherlock growing up to much. what happens when Desdemona comes to visit her siblings what happens after she comes back to London? How will Sherlock react after seeing his sister he hasn't seen since he was 12?

ok so this is my disclaimer I don't own Sherlock sadly but I do own Desdemona. This is going to take place at the beginning of season 3. I'm apologizing now for my grammar and punctuation. if I get anything wrong feel free to correct me because this is my first story.