Rick's a horny teenager and his Grandpa is a horny old man. When Grandpa arrives at Rick's house with a mind control formula, Rick's world changes for the better.
A family boating on vacation near a Virginia barrier island on the 4th of July are the victims of an unusual biological attack, with very deviant consequences. The last third of the story is resolution; all the sex is in the first two thirds. The coercion is biological, and reluctance or regret is brief.
Helen not only pursues her long fantasy of sex with her brother, she also pimps for him sometimes with their mother's three close firiends. Mom makes it a family threesome when outsiders are temporarily excluded.