Why is it that some people apologize profusely for little things but never apologize for things that are deeply hurtful?

One of my sisters has said some very hurtful things to me and has never apologized for any of them. But if she accidentally bumps into me, she will say, "Oh my God, I'm SO SORRY!" What's up with that? Shouldn't the apology be in proportion to the offense? Her behavior makes no sense to me.