Was death of a family member listed as an appropriate reason for a cancellation refund to collect on the policy? While that is appropriate to YOU, if that was not specifically listed as a qualifying reason....you have no grounds to complain.
I am sorry to hear your news. Your policy booklet will have a section in it on how to make a complaint and who to make it to. Firstly you have to give the insurance company a chance to fully explain the reason for turning down your claim. If their reason is valid (for example perhaps your mother was already ill with the illness she passed away from when you booked the holiday - you would not normally be covered for this if she was). If they cannot show you exactly where in the policy the exclusion which applies to your particular claim is, then you can submit a complaint as per the instructions in the policy.
Appeal to the insurance company first and make sure you have read all the small print to ensure the policy does infact cover you for the circumstances you experienced.
If you still get nowhere, consider a formal complaint to the financial ombudsman service.