"You could just refuse." - That depends. Your job might be on the line. You might have signed something when you took the job indicating that you'd comply with various employer policies (including policies relating to insider trading and conflicts of interest, as they evolve from time to time), so refusal to go along with a policy or revised policy could be grounds for termination.
"There are rare scenarios where I can see this level of disclosure as being necessary ..." Actually, in my line of work (which includes 30 years in financial services, financial services regulation, business law etc), I can see the need for this kind of disclosure if the employee has access to information, or influence on decisions, that either present a risk of conflicts of interest (in terms of decision-making) or insider trading or tipping.
"Policies ... can often be changed or at least circumvented." Well, circumventing a policy would be a bad idea. That could be grounds for discipline or termination.
If you think the policy is ambiguous or overly broad in scope, then it is worth having a discussion with the people who drafted it to get a sense of what was intended. You might have overlooked some language - and I have a feeling that the description you provided in your original question might have left out some information about how the policy works. If the policy is badly drafted, then you can provide input on how it could be changed, but keep in mind that, as I mentioned earlier, it's not that unusual for a policy like this to cover extended family members where the employee has knowledge of the family member's investments and/or influence over trading decisions. And if the company wants to keep track of extended family members' holdings, arrangements can be made to have the information provided directly by the family member's financial institution to the compliance department at your company (so that means you don't see the information).