This story is half-way based on history. Half-way because I had to change a few details to make it fit with the play. (Not that I'd guarantee that it does fit. I have a horrible tendency of misreading it, because
I HATE MACDUFF!!!
and so have a very pro-Macbeth slant in my reading. And writing. But I don't think that should be a problem here, since I see no reason to include old Tuck-Tail in this particular story.)
Anyway…
Because the history isn't exactly well-known, I'll give you a wee bit of background.
The big idea is that there are two sides of the Scottish royal family who both have claim to the throne: the House of Athol and the House of Moray. Naturally, they fight over who has the better claim, pretty much like in the War of the Roses.
So King Malcolm II, Duncan's grandfather, marries one of his daughters to Sinel, the thane of Glamis (in real life Findlaech, Mormear of Moray). He raises the son, Macbeth of course, in his court hoping, eventually, to use him to control Moray.
So Macbeth is from Athol on his mother's side and Moray on his father's.
You'll also need to know about Gillecomgain, Sinel's nephew. He murdered Sinel and took over his position. That was while Macbeth was fifteen years old and still stuck at his grandpa's – five years before this story takes place.
Gillecomgain married Gruoch, the granddaughter of King Kenneth III, who was from another branch of the family. Kenneth and his four sons were killed by Malcolm II, but she escaped.
That should be enough background. So, here goes.