Nephew Using Aunt to get to Mom
The basic idea is a Nephew seduces his young aunt (either when he visits her, or she visits her sister's)
After that first seduction she can not get enough, she comes to him willingly. He takes advantage of it, dominating her, making her his helpless slave.
The kicker comes when he uses his control over his aunt to get her to slip in into his mom's (her sister's bed) seduce her and get here ready to accept her son's cock.
That's the basic plot, that is.