Whether or not you're married, everyone will be happier if your partner's mother loves you. You can win your mother-in-law over by sending your love to her.
Choose a Friday evening when the moon is waxing. Take a pink candle and write your wish around it. Your wish should come from the heart. It could be something as simple as:
"(Mother-in-law's name), love me."
... I am so sorry to do this to you, but this post has been moved to my new website, Gypsy Magick and Lore, and can be found in its entirety here: Love of Mother-In-Law Spell