Unfortunately I think I would be right to assume that the paperwork you are refering to is the computer generated paperwork that the Life company send out to you and as such it will be based on what has been input into the application and this is what the Life company will base its' underwriting on.
I would follow Huckster's advice and follow through with the claim, if you give an insurance a reason to decline a claim they may well grab it and use it as an opportunity to save themselves a few pounds. If you do not mention it and the investigations do not come across anything that would affect the claim then it will pay out as needed.
If during the claim the cancer is noticed and questioned then you can play dumb and say it was disclosed at the time and it is an error on their behalf (not quite honest but then again it has been proven recently that neither are the insurance companies when it comes to claims (55 adjusted claims forms out of 100)
If you go down the honest route and claim it was an error then they may or may not decline the claim but it will definitely depend on the type of cancer the family member had. Normally when they are referring to cancer in family members it is for natural brothers and sisters, mums and dads and normally they only want to know about it if it was diagnosed before age 60 or 65. They never normally ask about any other relatives on application forms. Off the top of my head the risky forms of cancer for heriditary purposes are breast or ovarian cancer, bowel or colo-rectal cancer but I am sure there are a few other forms that create hereditary problems too and it may be worth investigating if the family member may have had one of them..
As Huckster said, I would not make them aware of the error unless they ask about it.
Good luck and best wishes for your husband