My sister is going away for a couple of weeks and offered to give me her (company) car to drive while she is away. She works for a small family business and there is no problem permission wise, but in terms of insurance, is there any problem in me transferring my insurance on to that car for a couple of weeks?
Although I don't know how your family business is organised or if you are an employee, I believe the insurers of the company car need to be informed and the insurance either suspended in order for you to do the transfer or to have a temporary driver included.
There may be restrictions in the policy e.g. only employees of XYZ Ltd can drive the car. I also don't know what your sister's role in the business is, but unless she is the beneficial owner she will undoubtedly need the permission of that person to make the proposed arrangement.