Whoa, that is really weird! I'd be annoyed if my sister had accepted a friend request knowing that history. Plus, if he's that obsessive/psycho back then, I would actually change your profile settings to 1) block his access to your photos 2) block his access to your wall posts and status updates and 3) DEFINITELY block his access to info. You can either specifically exclude him from everything or just limit all your info to ONLY FRIENDS (this assumes he doesn't accept your friend request).
If he does accept your friend request, you should still consider definitely excluding him. I reluctantly accepted a friend request from my old PSYCHO boss but excluded her from seeing anything but my Profile photo and lists of other friends. To her, my FB page was just blank as if I didn't use the site.
Anyhow, I still wonder why he friended your sister. Just weird.