Reply to her "I'm really sorry you feel that way. I love and respect you -- and your entire family -- and this is extremely difficult for me. I'm doing what I feel I need to do to protect myself and to fight for my marriage. I would ask that you support me in that effort, but if you feel you can't, I understand, and this will be the only response you'll get from me on the subject. Love, Formerly"
And then don't respond to her anymore. You don't need to continually defend yourself to her family.
Puppy
Yup. Your wife's sister needs to butt out, if she can't understand that you have found out why your wife asked for separation and if she can only see things your wife's way which unfortunately will probably be the case, end the contact with that message and continue doing whatever you need to do, don't let your wife's sister impede your actions.
