Sometimes it was a matter of being expected to do so, rather than allowed to do so. My Italian grandmother was expected to live with and take care of her deceased husband's brother. Fortunately, her mother (my great grandmother) said nothing doing and got her out of town. No doubt the social order would have survived if she had married her brother-in-law. But her life would have been a hell of a lot worse. And this is saying something, given how bad it in fact turned out to be.