See your $20/week to your sister as you repaying back your parents for the tuition, car payments and everything else you've received. Yes, it was their decision to help you out but you can't say it hasn't helped massively with your financial situation.
However, don't just give the money without any stipulation nor end date. Maybe make it $500 for every year she gets a certain GPA at college, or you (along with your parents) pitching in for her car payments in the future, or you (along with your parents) helping her save for a future holiday (say a european wide trip post graduation that she saves for and you will match what she puts in). Basically, show your parents that you are there to support, help and even, treat your sister but she also needs to pull her weight. By agreeing to one-off sums, you'll also wont be stuck giving her $20/week for years.
My family's finance runs similar to yours - me and my sister came out of university (with postgraduate degrees) with literally no debt. When I got my first job my sister paid for my deposit and first month's rent. When I tried paying her back my mom told me not to because my parents 'owe' me money (I've just paid for her and my dad's holiday) and my sister owes my parents money (she was living at home). i.e. the money will just run around exchanging hand within the family anyways. It's like when you go out with friends and you kinda owe each other change here and there (covering tabs, not having cash, etc) but you don't ask for it because you know it'll all even up in the end. It's like that for us - but with higher sums of money.
Of course, this all only works as me and my sister have roughly the same income, and we're both geeks in that we compete on who has saved the most money and save in the knowledge that we won't screw each other over money.
I dunno....maybe as we get older and marry off and have kids, some lines need to be drawn but I've taken so much out of the family money pot for schools and such that I'll take (and even make) any opportunity I get to pay it back (holidays, presents, etc)