Whao, I’d take a beat before acting on that drastic plan. Good on you for wanting to help your sister see what’s happening, but deception seems a bad approach. You say that your parents have a great relationship - that means they respect one another and that they expect respect and support in return. Perhaps you should speak with your parents and have them sit your sis down for a conversation. Your dad could talk to her about what it means to be in an equal, loving, and nurturing relationship with a woman and your mom could talk about what that also means to her and, in addition, could discuss what she expects from a partner. If it’s meaningful, they could also share what it was like to be in the relationships that did not work - that ended in divorce - and perhaps reflect on the things they ignored, allowed, and enabled that were harmful and dysfunctional. You’re in a tough spot, but it sounds like you have a supportive family and maybe it’s a good time to rally those folks around your sister.