Judging by the review on DMM, VENU-512 one of those movies set in a world where sex isn't some taboo thing but something just as normal to do with others as a handshake or something. In this case Reiko plays the married mother of a family that has sex with her son, with her husband's coworkers when she brings his lunch to work, with people she meets on the subway...
So it's not just incest, she basically gets it on with whoever she meets. The final scene has her doing her son while the husband (I assume) is doing the daughter.
reminds me of this movie, one of my first incest titles