Why? She's trying to spy on you and your life with dh.
Not sure about all the ins and outs of FB, but if you are linked with your MIL and the others, and she is linked with them too, can she see pictures of you and your immediate family? If so, I'd be blocking those photos.
Anyone remember the really old creepy movie where an old girlfriend worked her way into the family, killed off the wife and became the happily married new wife and mother to her kids? Not saying that could happen, but I sure wouldn't be opening any doors or windows to old girlfriends in the past. Sane people move forward with lives, they don't try to re-live the past.