Someone has used my iTunes gift card balance to buy Someone has put themselves as my family on my iTunes gift card account and used up almost all my remaining balance. Is there something you can do about this? This person is definitely not in my family or anyone that I know.
Contact apple support immediately. This happened to me (fairly recently) and they took care of it for me. I got all the money credited back to my account.

Also (this should be a given) make sure to change all passwords. Maybe consider two factor authentication as well (if not already enabled).