several other ideas to offer outside of these two but currently seeking: mother/son and father/daughter.
father/daughter - daughter just turned 18 and is in search of her real father. when she finds him, sparks fly, and she decides to wait on the big reveal to get to know him a little more. he doesn't even know he has a daughter, so when things get sexual, she's the only one that knows the truth. how long can she keep the secret? how will he react to the truth?
mother/son - son has decided to stay in town to get the first two years of college credits out of the way because he's worried about leaving his mom alone. one night, he hears something that makes him jump out of bed to investigate.. only to walk in on mom masturbating. he knows he shouldn't be as turned on by it as he is and that he should just go back to his room, but he doesn't, and from that night on he has a new way of keeping mom company.