Your sister is not required to have the car maintained by the Toyota dealer to keep the warranty in effect so that statement by them is bogus. She can have anyone service the car and keep the warranty valid.
However, she should maintain it at least on the schedule required by Toyota. Failure to do so could possibly mean a denial of a warranty claim later if Toyota feels the problem was caused by failure to perform a certain maintenance item on schedule. E.G., an engine problem develops and she cannot produce receipts and a log to show that the engine oil was changed on a regular basis.
About all I can recommend is that you get a manual and follow the steps for troubleshooting the evaporative system. It’s normally not too complicated but a methodical process should be used. Wild guessing here but the problem is usually a cracked rubber hose (would be unusual on a 40k miles car) or a defective purge solenoid.
Something to keep in mind here is that your sister should not be in the habit of topping off the gas tank when getting fuel as this could damage the evaporative canister. Round it up to the next dime or whatever but don’t continue to force feed gasoline into the tank until it’s near overflowing.