Engaging a child of this age can be very difficult and will take a lot of time. Two basic things that would help are to find an interest or hobby to do together that will help them to relax. Find out what they are interested in, learn about that topic and chat to them about it.
The second strategy would be not to ask the teen any questions at all, to chat a lot, be very friendly, and use humour and silliness to engage them. Don’t expect a response right away; you will need to be patient and persistent.
I would also be encouraging the family to seek treatment from a Psychologist to work on their anxiety and communication.