First of all OP I would thoroughly read the insurance policy to make sure that you are indeed comprehensively covered for whatever the purpose you use the car for.
If you are able to contact the man who has kindly lent you his car, then I would get in touch, and let him know that in spite of him saying he didn't want any money, you felt it wasn't right to be sponging off of him, and are just incredibly grateful that he's letting you use his car, so you have given his sister £400 to cover the tax.
Then, as this obviously isn't sitting well with you, I'd see if I could manage to get myself a small run around, and hand the car back.
If it isn't possible to get yourself a car of your own, and you can't contact him about the money you've already paid his sister, then I would put all further sums that you feel obliged to give him, into an envelope, together with a note saying that you gave the first £400 to his sister direct, and put the envelope in his hand when you give him the car back. (DO NOT give it to his sister to give him!) Then if he truly doesn't want or need it, he can put it back through your letterbox, give it to his sister or even the cat's home, if he wishes, but at least you will feel that you haven't been a CF, as MN is usually so happy to call people who don't pay their way.