Reading the instructions, I would certainly agree with you that it appears we should be labeled a Category 5 filer.
But the Form 5471 was prepared by one of the Big 4 accounting firms. They insist on filing it for each of the shareholders which I will gladly accept. But they checked Category 3
And I found this on a Top 60 CPA firm's website
Category 3
This category includes:
Any U.S. person that acquires stock in a foreign corporation that would put the person above the 10% ownership threshold (vote or value)
A U.S. person that acquires 10% or more of the foreign corporation during the tax year in addition to what they already own
A person treated as a U.S. shareholder in a captive insurance company
A person that becomes a U.S. person during the year while meeting the 10% ownership threshold
A U.S. person that disposes of stock and reduces their interest to below the 10% ownership threshold during the tax year
There is some nuance here – the 10% ownership threshold looks at what the U.S. person owns directly, indirectly, and constructively. However, with respect to individuals, the constructive ownership rules are expanded to include brothers and sisters. Thus, for an individual, this category is applied via attribution from spouse, ancestors (parents, grandparents), and lineal descendants (children) as defined above, but it also includes siblings (brothers and sisters) for purposes of Category 3.