Your mom catches them fucking and joins them to make a threesome

When their mother catches them fucking these brothers, they think their incestuous fucking adventures are over forever. And while at first Mom fights well and stretches her ears so they can see how they behaved, the mature woman sees an opportunity to have a good dose of sex, because her husband hasn't fucked her in a long time. He tells them that if they fuck her, she won't say anything either and let them go their own time as long as they want and fuck as many times as they want, so they end up making a spectacular trio.