I'm double-jointed too GC and my rheumy said that a lot of my joint pain could be attributable to hypermobility rather than arthritis (
https://en.wikipedia.org/wiki/Hypermobility ), which can cause a lot of joint pain, among other things. My joints also all crack pretty much every time I move (my body thinks it's 30-40 years older than I am). I think hypermobility is the only thing out of all my medical issues that someone else in my family has (my dad).
I can pretty much do exactly what that hand is doing the the picture on the Wikipedia page. :)