When you will tell your wife about her sister is spying on you. Your wife might not trust you fully at first. But, this will create a space in her mind to find out the real truth. According to you, your wife’s sister is spying on you. Then there must be a reason for doing that. Maybe she doesn’t trust you. She might be just protective of her sister. So, try to talk to her before going to your wife. You might get to know why she is doing that. If you won’t be able to get a conclusion. Then only talk to your wife about this. Try, to collect any sort of evidence so that your wife will believe you.