I still say that you do not have to driver her. You are already driving 2 1/2 hours (give or take) to go to BIL's house and cooking food to take.
If your brother comes and gets her and takes her back, he will also be driving 2 1/2 hours (give or take). Why should you drive another 1 plus hours (total now of close to 4 hours for you!) so your brother only has to drive 1 plus hours???
He invited her, he should be the one to get her and then bring her back home; it does not matter that she lives close to you. You have other plans and should not have to change them. What would your brother have done if you were out of town those days? He would have had to get her then or leave her at home.
If one or the other calls again, simply reply "The answer is no, it is not possible." Then change the subject. If they call again, "I have already given you my answer. Can you believe how warm (cold) it is today?"
Remember: Poor planning on your part does not constitute an emergency on mine.