Have you read the revenues info on Capital Acquisitions Tax? Its all in there.
No tax implications for you - any implications are for her.
Sister is Cat B, Allowance 32500 tax free lifetime allowance presuming this is your first gift. Then you have the 3k annual allowance from you to her (and your partner to her if you have one). If you have a partner this is past 38K. If not then technically she has 35500 this year tax free and would pay CAT on the rest. This, or any amount of the 38k, could be done as a loan to avoid the tax as long as some agreement done up.
You could buy car at market value (with a generous valuation) but any ridiculous valuation would be seen liable for CAT if over thresholds.