User blog comment:Timelordpinks123/Doctor who, master and doctor brothers?/@comment-13187859-20141220005804

Funny story, I was talking about this just today. As you said, it was their intention during the Third's era, but they dropped the idea after that. I just thought the reference with Ten was just meant to be a sly nod to that idea. And, uh, considering the way that the Master/Missy talks to the Doctor in series eight, I'd assume they couldn't be related. Because that would just make things even stranger than they already are...

My vote goes for no, they aren't related, but you can do whatever you want with this, Pinks. They're your characters, after all. ^^