Wow, this was a tough one!
It seems that the author's last name was "Caute" rather than "Choate."
The book was "Veronica: or The Two Nations" (1989/1990) by David Caute.
"Michael is almost 12 when Veronica, introduced to him as his cousin,
comes to live with the family. She is 15, mysterious, beautiful, and
indulgent toward Michael's awakening sexuality. By the time he finds
out she is his half-sister, he is too far gone to care. Forty years
later the youthful illicit relationship threatens to ruin Michael's
successful political career."
"From Publishers Weekly...
Michael Parsons falls "instantly in love" with his South African
cousin Veronica when she comes to London in 1939 to live with his
family. By the time his parents die in WW II, he knows that V is his
half-sister. He justifies his passion via entries in his journal that
record historical incidents of sibling incest and its endurance as a
classic theme, copiously citing the Jacobean tragedy 'Tis a Pity She's
a Whore and Ibsen's Ghosts . Determined to destroy his rival, V's
American soldier fiance, Mike stoops to deceit and trickery. When V is
hurt in a bomb raid and becomes dependent on barbiturates, her
inhibitions abate and her mind deteriorates. Mike tenderly cares for
her as their love flowers. Years later, Mike, now a Tory cabinet
minister under Thatcher..."
More used copies at abebooks.com:
abebooks.com advanced search: author: david / title: veronica