When — even when you want to go to your friends' party — you're incapable of telling your mother you are going to miss her birthday dinner, it means you just can't do it, and that you're a good person. Incapable means "unable."
If incapable means "unable," does capable mean "able"? Yes, but capable also can mean good at getting things done in general, while incapable less often has such a general meaning. You would say, "I'm incapable of making a decent loaf of bread." You'd be less likely to say, "I'm incapable in the kitchen."
