It's another bloody Facebook one! Apologies in advance 
I saw on Facebook this morning that SIL and her husband have become friends with my DH's ex of over 10 years. I know this because it comes up on the newsfeed with a 'X and Y are now friends' type message. SIL and her H haven't seen ex in at least 10 years and have no relationship with her whatsoever outside of the fact that she once went out with my now H. H and I both think this new Facebook friendship is quite odd. Neither SIL or her H are the type of people that have hundreds of Facebook friends.
To avoid a massive dripfeed, Ex has a serious mental illness and is unstable. She has done various odd things over the years including, attacking and burning H's girlfriend before me with a cigarette, getting upset at a mutual friend's wedding and asking H to take her back 5 years after they split, and writing about him on her blog as recently as 6 months ago.
SIL puts a lot of photos on Facebook of H and I and our family. AIBU to feel uncomfortable that ex can now see this? I would prefer it if she didn't know anything about my life at all given her previous behaviour.