My sister stole my dads savings and our inheritance. What can we do about it?

My dad is 84 years and of sound mind. He had £8000 in savings and in his will stated that each of his 4 children will get £2000 each. She has cleaned out his savings. We found out and have proof that it was her and she admitted it. She has paid some of it back but still has a few thousand to go. However, she has stopped paying and wont answer calls etc. My dad doesnt want to go to the police after all, it is still his daughter. He wants me to get Power Of Attourney as I deal with all his finances now. Is there anything we can do to get the money back?