Duncan, Macbeth, and Thorfinn first cousins?

["John S. Quarterman" <jsq@quarterman.org>]

I'm puzzled by the comment that Macbeth, Thorfinn, and Duncan were
first cousins.  It made sense when I first saw it, but now I can't
figure it out.

Can anyone elucidate?

John S. Quarterman <jsq@quarterman.org>

>   [1]Tomorrow:
>   August 14
>
>   1040:  Macbeth,  King  of  Scots.  Macbeth replaced Duncan as king. In
>   spite  of  Shakespeare's  later bad press, Macbeth was actually a good
>   king;  well,  good  as medieval rulers go! He ruled well from 1040-57;
>   vacationed in Rome. After Malcolm II's murder by his nobles at Glamis,
>   Duncan  killed  his  opponents  and seized the throne as King Duncan I
>   (1034-1040).  His  first  cousins, Macbeth (of Shakespearian fame) and
>   Thorfinn the Raven Feeder, Norwegian Earl of Orkney, united to advance
>   Macbeth's  claim to the throne through his mother, another daughter of
>   Malcolm  II.  Duncan reigned from 1034 until he was defeated in battle

[ Excess quotations omitted. ]

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