The friends thing sounds just like situations I used to have. Before we broke up from school they'd always say how much they'd miss me and wanted to meet up etc etc. Once it actually got down to the holidays nothing ever happened. I wouldn't call them real friends to be honest.
I think you're doing the right thing in trying to back off when this girl comes to you for a shoulder to cry on. Don't give in to that. Obviously she's in denial and thinks the world of him.
I would give a flat out no to your family if they ask you to do stuff whilst you're doing A Levels. Or I'd say that you'll simply do what you can and leave it at that. You can't do much more than that, cos yeah you will be busy. They'll just have to accept that.