A guy gets divorced and asks his ex-step-sister to help him raise his two young twin daughters. Several years pass by and he gets confused by these sex dreams he's having of his "sister" especially since they feel so real. What he doesn't know is that while he's sleeping his "sister" is having sex with him. Eventually, he comes home from work to find his "sister" naked on his bed masturbating. He loses his restraint and has sex with her. He's partially disappointed that she wasn't a virgin. For a while, he feels guilty about what they did but the more they have sex the less guilty he feels. One day he's surprised to find his "sister" is the one feeling guilty.
She tells him she's pregnant with his baby. He's thrilled but she still seems to be feeling guilty. That's when she tells him that before he walked in on her that one time she had been sneaking into his bed at night and having sex with him. He's a bit shocked at first. Then she tells him the first time had been when their parents were still married and she had lost her virginity to him.
They eventually find out that she's going to give him another set of twin girls.