Dude.
Just, dude.
Please no.
Just No.
As others commented, she is a minor.
And when someone says they can't do anything but sleep with you to pay you back (because they think they are useless)... You don't sleep with them, man.
You would just be confirming to them the only thing they are good at is being a plaything.
What you do is tell them they might have something they are good at and they just didn't think about it yet, and even if they really have absolutely nothing they are good at (absurdly unlikely unless she was in a comma...), they can always learn something new.
She has two eyes, two hands, two feet, looks perfectly healthy both mentally and physically (even if not, the majority of people with disabilities can do most things people without those do with a proper adaptation...). Just get her a cloth and get her to wipe your floor, dumbass.
If you think you're helping someone when you sleep with them because they feel unworthy, I have some news for you, mah friend.
You aren't FREAKING HELPING.
You're not.
You're just making them depend on you and useless on their own.
Besides, she will just be a hold-down if her only utility is sleeping with you.