I think the inheritance angle is the best. Maybe the father was a scrape-grace younger son with no prospects, which is why Keltos opposed the marriage. But then the father's older brothers died, and he inherited, and the granddaughter inherited in turn.
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Actually, now that I think about it, this kind of reminds me of the plot to Mary Balogh's Wescott Series (Someone To Love):
The father is the son of an Earl, and marries a woman from the merchant class. He abandons her, and she has a daughter. The father marries another woman at the Earl's behest, but crucially he does so before his first wife dies. That technically makes the second marriage bigamous, and all issue from that marriage illegitimate. The daughter is the legitimate heir to all the non-entailed wealth. She grows up in an orphanage, and doesn't find out about her heritage until she is twenty.
You could tweak this, and have Keltos realize he can control the wealth of the Earldom if he has the granddaughter. And he could be opposed by the family of the second wife, who realize she and the children of the second marriage would be disinherited if the first marriage came to light.
Another enemy/rival might be the man who stands to inherit the Earldom if the other children are disinherited, especially if he gets to be named guardian of the granddaughter.
(Balogh's story is a bit different in that everyone involved is basically good, even though they are all affected by the legal ramifications of the bigamous marriage.)
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Actually, now that I think about it, this kind of reminds me of the plot to Mary Balogh's Wescott Series (Someone To Love):
The father is the son of an Earl, and marries a woman from the merchant class. He abandons her, and she has a daughter. The father marries another woman at the Earl's behest, but crucially he does so before his first wife dies. That technically makes the second marriage bigamous, and all issue from that marriage illegitimate. The daughter is the legitimate heir to all the non-entailed wealth. She grows up in an orphanage, and doesn't find out about her heritage until she is twenty.
You could tweak this, and have Keltos realize he can control the wealth of the Earldom if he has the granddaughter. And he could be opposed by the family of the second wife, who realize she and the children of the second marriage would be disinherited if the first marriage came to light.
Another enemy/rival might be the man who stands to inherit the Earldom if the other children are disinherited, especially if he gets to be named guardian of the granddaughter.
(Balogh's story is a bit different in that everyone involved is basically good, even though they are all affected by the legal ramifications of the bigamous marriage.)