Obviously you think this would be upsetting to your wife as you talk about what steps you can take "to ease her into it." Why would you want to do this? Aren't there enough other females in this world to choose from for a poly partner(s) such that you don't have to complicate or possibly compromise your current marriage and family relationships?? And assuming her daughter is an adult.........has her daughter expressed an interest in "polying-up" with you or anyone else for that matter? If you're serious about this...and not a troll...such a discussion should occur between all three of you as equal adults, not "daddy" asking "mommy" if he can "poly/play" with her daughter!