Isis, although the original definition of a cuckold was a title reserved strictly for married men who's wives were having sex with other men, the definition and usage of the word has broadened over the past decade or so. You might recall Sandra Bullock and Elizabeth Edwards, who both publicly referred to themselves as being "cucked" by their respective husbands after finding out their husbands had cheated on them. Cuckolding has become more of a fetish practice among couples than an act of infidelity by a married female. Thus, both women and men (married or in unmarried, monogomous relationships) tend to use cuckolding as a way to create an added sexual stimuli to an otherwise boring sexual relationship.
Today, cuckolding is more similar to swinging relationships, except in the cuckolding relationship only one partner (usually the female) has extracurricular sex partners while the othe partner (usually the male) remains monogomous to her. And often, it is now the male, of the relationship, that encourages the female's infidelity ... another word for him might be "wittol".
The fact, however, that the stage is being set by your sister for you to start fucking her new husband tends to hint of a dom-sub relationship, or master-slave type relationship. If this is all in agreement with everyone ... no harm, no foul. If, however, your husband has issues with your fucking your sister's husband, then a better understanding of the terms should be worked out, otherwise one, if not both, relationship is bound to eventually be doomed. Hope you'll keep the forum posted on your adventures and development. Mac