Since this thread is a safe anonymous environment, you can experiment safely and hopefully learn something about your motivation and the possible outcomes of alternative steps you might take.

Say, you will reveal the secret to your sister-in-law. How exactly will you go about it? What will you tell her? In what room will you be when you tell her? Or will you invite her out and tell her somewhere else, outside her house? Which words will you use? In what tone of voice? How will you even begin this uncomfortable convo?

If you tell us specifically what is playing in your mind in terms of how you would wish you told her, we might give you feedback in terms of how your sister-in-law might react. Then you can think of how you would handle alternative possible reactions coming from her. Etc. I think it would be a useful exercise for you. At the least you won't feel so alone.