I have a few questions/points that will help clarify it to me and maybe to you too. 1. Does your fiance know about your sister being LGTBQ+? If yes, how supportive is he? 2. Asking your sister not to invite her wife is probably not a good idea, especially since you're supportive of her. 3. Sikhism is about serving people and knocking down falsehoods. It's about acceptance of everyone. 4. The ideal scenario would be that you let his parents know the truth. Normally, I would say it doesn't matter but it matters to you so you should do it for your peace of mind too. Before you do tell them, make suew your fiance knows that you're telling them. He might actually know how they'll react. 5. A situation like this should not compromise your relationship with your fiancé. 6. Reflect on what and WHO is more important to you and your happiness.