Does make sense. How far back do you want it to go, and would you prefer a male or a female connection? If male, would you prefer that your lady marry the titleholder or a younger son?
Regardless, here are the possibilities.
Jonathan Edgeworth (1686-1752), 7th Earl Saxton. (Tristan and Cassie's great grandfather)
Thomas Edgeworth (1719-1787), 8th Earl Saxton (Tristan and Cassie's grandfather)
Adrian Edgeworth (1760-) Current Lord Saxton's brother. (Tristan and Cassie's uncle)
More can be invented if necessary.
Dates of birth and death can be tweaked if need be. Nothing is set in stone so far.
Julia Edgeworth (1756-), Current Lord Saxton's sister. (Tristan and Cassie's aunt)
Mary Edgeworth (1722-1760) and Katherine Edgeworth (1725-1799), Thomas Edgeworth's sisters.
If anyone else wants to pick up some of these relatives, feel free to do so. Be advised that Mary Edgeworth was a scandalous woman though!
Should someone want slightly lower connections, Tristan and Cass' mother Susanna comes from a baronial family, the Greys. Her brother Gabriel, 5th Baron Melbury (1764) and needs a wife. Her mother, the wife of Christopher Grey, 4th Baron Melbury (1723), is also yet to be named and can be claimed by anyone. Beside that, there's a range of female Greys who would have been very happy to marry into some of the board's families.