Got a FaceBook request from YSD today
I thought one day soon that may happen
I've never mentioned to her I'm on FaceBook and I think she assumed I wasn't. DH's sister knew that I was because she rudely asked me one day and although I confirmed I was, she never mentioned it again let alone send a friend request LOL....however I do suspect 100% she snoops to see what I post
YSD is also Friends with my nephews etc on Facebook, not that they are on it much or post, and I thought about connecting with her in the past but then thought no, better to not put her in a position where she feels she has to accept but may not want to be friends with her SM on FaceBook haha
Anyway, for me not to accept her friend request I know she would be offended. And, since I have a great relationship with her why on earth wouldn't I. I was quite pleased about that today, and gladly did 
What's funny is now not only DH's sister, but OSD, will know I'm on FaceBook and friends with YSD. I doubt they will be happy about that/with her
Will be interesting to see if they continue to ignore that/me, or if they pressure YSD into unfriending me, or use it as a tool to get to me
Should be interesting
You can change your settings
You can change your settings so only friends can see you stuff. So add the younger sd and then swap to ensure only friends can see your site. This should reduce her snooping.
There is also a thing to let you see who has been on your page I think. You can also block people you don't want to see your page. So if you check and continuously see your sister in law on , show it to husband, keep a note of it in print form and block her. This would completely stop her snooping but you need to make sure your husband knows your side if you do.
Also don't put anything on Facebook you don't want the world to know. Make sure they have nothing to hold against from now on
Good advice Amcc13...how do I
Good advice Amcc13...how do I find out who has been on my page? Would love to see that!
I've been friends on Facebook
I've been friends on Facebook with my SO's teen daughters, their mom and even their mom's mom (grandma). I love being able to keep updated on the kids' activities and pictures and to tag them in any pics we take too. I don't put anything on there I wouldn't want them (or my own kids) to see so I don't worry about snoopers much. Its worked out so far aside from a couple comments their mom took a bit of offense to. Once I explained that she misunderstood all was well again :).
I say no harm done if your ysd and you have a good relationship unless you have something you'd rather not have her or her family see.