How to help my 13 yr old fall asleep?

[american_mommy]

I was reading the melatonin thread so thought I would start a new one with my question.
My 13 yr old dd has never been a great sleeper. But since she turned into a teenager it has gotten even worse. she will not fall asleep at night and part of it is her fault but she also told me she can't fall asleep. She has a bit of OCD and obsesses on things, thinking about them. I have given her a homeopathic remedy(Rescue Sleep) and it used to help her, it just helps calm the mind, but it doesn't seem to be working anymore.
Someone suggested melatonin but I am very hesitant to try it. And after reading one of the posts in the other thread I am more hesitant.
She has gotten to where she is affecting her 11 yr old sister who she shares a room with. They both stay up talking or playing around and won't go to sleep so they are exhausted the next day. Oldest gets super moody and 11 yr old has tantrums when she is overtired.
I just don't know what to do. Separating them isn't an option. DD knows she needs to be sleeping but she just won't. She is so tired she is extremely moody and gets sick more often. Any thoughts?