The most common cause of marriage problems with a sister-in-law is jealousy of the relationship you have with your spouse. Because you’re a threat to a valued bond, she will do anything to protect that bond by making your life miserable. Do not allow your sister-in-law to interfere with your marriage--learn how to play the game.
The Sister-in-law Plan
Sister-in-laws can be nasty for reasons we may never understand. The strategy is to take away their power. The best way to do that is to kill them with kindness. Smile at her every chance you get. Compliment her so much that she forgets why she was being mean to you. You can never be too kind. A cruel sister-in-law can’t defend herself against unexplained kindness.
The Sister-in-law Type
Maybe she is the kind of person who wants all the attention to herself and cannot stand when your spouse praises or gives you some attention. Letting her know she is a smart and talented woman is a "kill her with kindness" ego boost, especially if she felt growing up she was the loser in the family. Also, asking for her opinion, like you really care, is just the driving force to steer her mad.
Perhaps she ignores you when you talk. She says negative comments and talks rudely about you. Smile at her when she is calling you names. Being cheerful every chance you get and showing your sister-in-law that no matter what she does or says does not have an effect on you will drive her insane. If she’s spreading awful rumours, still kill her with kindness--the truth will eventually come to light.
The Sister-in-law Game
Think of dealing with your sister-in-law as a game of chess--it is you who control the pawns, how they move and what happens to them. Once she sees you’re a worthy opponent, she will give up. She will finally realise that not all the cruelty in the world could take you down.
Play the game by your own rules. It really is that simple. Nothing your sister-in-law says or does should be able to upset you, but only you can make the change. Once your sister-in-law perceives you as a threat, you have just captured your Queen. Check mate.