If you are changing the title from You-OR-Her to just You, that may be considered a sale. If your Sister is OK with it, I'd just leave her name on it.
Edit: If Sister needs to get your car off of her credit report, then you'll probably just have to suck it up and pay the sales tax.
I'm guessing that a change of owner on the title would be considered a new sale and be taxed even here in AR.