Sis in law from hell?

What a disgusting lady! Anyone in my family would frown upon such behaviour. It's no wonder she has this kind of attitude, since her parents too didn't care much about it. Everytime my partner forget to flush after using the bathroom, I'd be nagging til his ear bleed. To me, this is totally unacceptable, not to mention unhygenic. I pity you. It's good that you have moved out, and live life without sufferings. Try not to think about her habits anymore, and move on with your life. Be thankful your wife isn't like her sister. Haha! If she ever comes over to your house and use your bathroom, remind her to flush after use. This'll make her go red and blush (maybe), and when she's coming out, ask her whether she flushed or not. Treating her like a 5 year old girl will make her the joke in the family and sooner or later, she'll get rid of the habit.