From what I'm about to post, I've clearly spent way too much time looking at this stuff. Apologies in advance if this is a case of too much information.
Prior to the Interregnum, there were 5 Haldane-Furstan marriages, 3 of which had significance to the Haldane bloodline:
-Augarin's son Bearand married Kypriona Furstana (son, but no grandchildren)
-Augarin's daughter Betradis married King Aldred Furstan
-Augarin's son Aidan had 2 daughters marry Furstans
-Bethany married King Imre Furstan (son of Aldred and Betradis)
-Moira married Aleksy Furstan
-Augarin's grandson Ryons (by son Llarik) married Raphaela Furstana (daughter of Imre and Bethany)
And Cinhil was Ryon's and Raphaela's great-great-great-grandson.
An interesting topic of speculation: how much power those Haldane's might have had, with or without training. I'd agree it a logical way to explain Cinhil's genetic receptivity to the powers bestowed on him.
And while these Deryni marriages probably didn't contribute much to Kelson, after that many generations' remove, there have been a handful since that may have refreshed what was there before:
-Michaela Drummond married Rhys Michael, as previously mentioned. Neither the Drummond or Howell familes were particulary strong strains of power, but there was at least something there.
-Uthyr (son of Rhys and Michaela) married Grania MacInnis (daughter of Iver MacInnis and Richeldis MacLean. Granted that Richeldis' power (what little there was) was blocked in adolescence, but I recall Joram and Co, believing/speculating that blocked parents would still transmit the power down to their children, so Grania may have gotten a tiny bit from her mom.
-Nygel (son of Uthyr and Grania) married Susanta Howell (Sudrey's great-granddaughter). No surviving sons, and even Sudrey's daughter didn't have hardly any powers other than shields, so moot point with this marriage.
-Cluim (son of Uthyr and Grania) married Swynbeth FitzArthurQuinnell, with ancestry listed above. Likely a powerful jolt since it traces to Camber.
-Brion (Cluim's great-great-grandson) married Jehana, which is logically the primary source of Kelson's non-Haldane powers.
-And since Brion's son Kelson (with his dual powers) has married Araxie (also with dual powers, or at least the potiential to have both), their children will undoubtedly be formidable.