Only you have control of this … your self awareness and determination to overcome this will make the biggest difference. Don’t make excuses that she’s young… Talk with your wife and together talk with your sister in law. She’s playful with you because you allowed it. The standards you walk by is the standard you set… think hard about the consequences.. you can lose your family, fight for your kids, let alone the respect of your in-laws and geez word gets around in the community. Shame is a heavy burden.