1. The Sage cries Poppycock! You have enough genetic information about your family from your father and siblings, which is more than many people have now, and as much as most people had in the early 1950's. Admit to yourself that you are simply overly curious. Feel free to test yourself for the gene carrying your niece's disease, but respect your father's wishes and make no inquiries as to his origins until his hopefully distant demise.
2. The Sage would point out that whether your husband flushes or rinses, water is still going down the drain when he urinates. This is a disgusting and juvenile habit which he needs to stop right now. Purchase him a male portable urinal and suggest he carry it around, and once he pours the full container into a toilet, to feel proud that he saved the water for a couple of extra flushes.
3. The Sage will not give you permission to harm this woman farther. She either does not know of the affair, or more likely, has accepted that her husband was unfaithful and has put it behind her. Telling her about it now in the form of an apology would serve no usefull purpose. Speak to someone to help you get over your obsession with this man, made evident by your compulsion to search the Internet for information about him.
4. The Sage suggests that you invite this friend and her fiance to a dinner with only a few people she actually knows well, preferably just you with your partner and she with hers. You admit that she is very shy. Having a fiance does not change her character; it only provides additional support from a loving man when she does find herself unexpectedly out of her element.