Your SIL went overboard. It is not in her place to trim your child's hair, nor to take him away without your agreement. You're the mother, and I daresay, have even more authority over the care of the child than even your husband. Suggest you have a good and serious talk with your husband on her overstepping her boundaries, and see if your husband agrees with you, and whether to talk to her. The talking part must be left to your husband though..as the in law, it is usually a bit more sensitive. If she doesn't change after talking, then look at reducing contact with her. Now is cut hair, next time you dunno what else she'll do. You're protecting your child, heck if she's offended.