Long forgotten connection

Not so long ago, I got a "friend request" on Facebook from someone I didn't recognise.  I looked at the face, and I looked at the name, and I puzzled over it.  After a while and some digging through their profile, I finally worked it out -- this girl was my brother's ex-girlfriend.  From about 15 years ago.

I have no idea if my brother still has contact with this girl, but I expect he doesn't -- he isn't big into social networking, has limited contact with people from his past, and has long since moved away from our home town.

I found it more than a little creepy -- I mean, sure, we were quite close once.  When I was 15.  I think everyone drifted apart and found their own lives when my brother and this girl went to seperate universities, and I don't think it's unkind to say I've never really given her that much thought since.  What made it particularly weird for me was seeing that she had already added my parents as friends -- I wanted to tell her not to do that, to leave them in peace, they probably don't know that you aren't obligated to accept every request you receive.

Shunning her request, I thought no more of it.

Skip forward a few weeks or a month or however-long it was.  Another friend request turns up, from a girl named "Kate".  Following directly on from the weekend when I'd been talking to a friend and his fiance -- who is named Kate -- I accepted without thinking.  Then I noticed the little details -- about how our only mutual friends were my family, and I realised I'd accidentally accepted the long-forgotten sibling's ex.  Figuring it would be mean to delete her again, I moved her to Limited Profile so that she'd have restricted access to me and my life.

Seems that wasn't enough, since accepting her request was apparently like saying "please contact me further" and she sent me a long email.  Not much of it stands out, apart from the bit where she mentions having emailed my brother but not got a reply, and how she'd been looking at the photos of his boy.  I still find the whole thing more than a little bit weird.

I'm currently in a dilemma, despite all of the above.  I feel bad for not replying.  I know that I'm slightly offended when my emails go completely ignored, so part of me wants to send something -- however brief and short on details.  But the other part continues to insist that to reply will only encourage her -- and whether deliberate or not, my brother's tactic of not responding might be a better one.