Ask the lender if they are willing to add your name to the loan or refinance the loan with your sister so both of your names are on the loan. Either way, a credit pull will occurr.
It sounds as though you did not really buy the car, your sister did. The lender does not really care who is making the payment, but it would be your sister who may have been gaining a few points on her credit score and it is her the company would demand payment from should you stop making payments.
As was said, if you and your sister could refi the loan together then that may solve the problem, although your sister will still be on the hook for the money too, should you default. Another issue that could arise is if your sister ever needed to purchase another vehicle. Since this loan is on her credit reports she may run into trouble getting another loan in her name.
And of course, doing a refi may depend a lot on if you are upside down on the loan.
If you can qualify for the loan yourself then you can simply buy the vehicle from your sister and use whatever lender you like.
Just one guys opinion. Good luck with it
^^^Chris in Tx is right. Either refi it with another lender on your own or with your sister. It would be best if you could do this on your own rather than tying up your sisters credit line (it at all possible).