When you married your husband, you knew he had a sister. What you didn't realise is that his sister, and your new sister-in-law, is a busybody who shows up unannounced several times a week and stays well beyond her welcome. When dealing with your sister-in-law, have a serious discussion about your grievances. This will help you to take the necessary steps to keep your sanity intact.
Determine the exact points that cause you stress with your sister-in-law. It may be that she shows up during late hours or that she calls 10 times a day to give you unwanted advice on how to raise your children. Write them down so you have a tangible list as to what your grievances are.
Discuss the issues with your husband, her brother. Give him specific examples of your frustrations. Be sure he is on board. It's best to have him in your corner when dealing with your difficult sister-in-law, so there are no surprises. Decide whether it's best to have your husband speak to her or if you should speak to her with your husband's blessings. You may decide it best to have both of you speak with her.
Invite your sister-in-law over for a non-confrontational discussion. If you speak, be sure your husband is in agreement with you. In a non-confrontational manner, tell your sister-in-law your issues. State your boundaries. Negotiate visits, if needed. For instance, have her come by two days a week instead of six. Have your husband interject with his point-of-view to demonstrate that this is a concern for the both of you and not just your problem.
Allow your sister-in-law to speak her point-of-view so it doesn't look like an attack. End the conversation in a civil manner. For instance, if you feel comfortable doing so, invite her to lunch next week to show there are no hard feelings.
Don't be afraid to remind your sister-in-law of the set boundaries if she falls back into her habits. Just remember to be non-confrontational. Pick your battles. It's not a big deal if your sister-in-law stopped by unannounced one time in several months. Small infractions of the rules are not worth the headache. It should only become an issue if it occurs continually.
Never force your husband to choose between you or your sister-in-law. This puts him in an uncomfortable position and may cause friction in the marriage.