#1-God does not ALLOW bad things to happen, but he lets them happend to see how we handle them!
#2-you don't say how old you and your sister are but I assume your at least 16 if your old enough to drive. Are you the 2nd oldest?
It sounds to me like your sister is not taking responsibility for herself, she knows that if she don't you will, it also sounds like she might be a little on the self centered side, she thinks it's all about her with no consideration to the rest of the family.
Try talking to your parents about your feelings, if that don't help, go to your school counselor or a friend and talk to them, alot of times talking helps. Another thing, write her a letter! Tell her how you feel and how what she's doing makes you feel, I know for me, putting my feelings on paper is easier then trying to get them out verbably.
I'll be praying for you and your sister! GOD BLESS!