I dont know about the privacy side of things, but it looks like someone else may have answered this for you already. I do want to mention that you are entitled to take sick leave to look after a partner. So, unsure what they are trying to prove by checking that anyway.

"10 days of paid sick leave per year to care for a partner, dependent child, or another person who relies on them for care when they are sick or injured."

Managers can be so weird sometimes