Try the laundry soap you use to regularly clean the sheets but in a little more concentrated form. If that does not work, try ammonia but be careful if the sheets are a dark color. Test in an inconspicuous place first and use as little as ammonia as possible. Start with 1 part ammonia to 3 parts water. If that doesn’t work increase the strength of the ammonia but never use anything stronger than 1 part ammonia to 1 part water.
After all, care for not “kiss” your silk bedding that would be the best solution.