Greg responds...

Well, they're all related, at least distantly.

But here goes...

Kenneth I (the first high king of Scotland) had two sons Constantine I and Aodh.

Connie-1 begot Donald II. Meanwhile (to keep our generations straight) Aodh begot Constantine II.

Donnie-2 begot Maol Chalvim I, while Connie-2 begot Indulf. (Up to this point, NONE of these are people we've met in the series.)

Maollie-1 had three sons: Duff, Kenneth II and [the fictional] Malcolm of Wyvern, while Innie begot Culen.

Kennie-2 (Katharine's uncle) begot Maol Chalvim II (Katharine's cousin) while Malcolm of Wyvern begot [the equally fictional] Katharine... and while Cullie begot Constantine III. (All of these characters, except Cullen, were featured in "Avalon, Part One".)

Maollie-2 had three daughters and no sons.

The eldest Bethoc begot Duncan I (from "City of Stone"). The middle daughter Doada married Findlaech and begot Macbeth.

It's easier to see on a chart. But hopefully you can make your own chart with the info provided.