Many women do more than that when they're alone with themselves. But the contrived rules do not allow them to relax with a partner. It is not without reason they say, that a smart woman has it in her head, a fool has it in her mouth. I even know men who categorically reject such liberties.
Brother does not have to get bored in the family circle: while he puts his little sister on his dick, while the second one, the evening will pass imperceptibly. A threesome is much better than television and is healthier!