Their policy still does not cover your things
Whether the owner is a stranger or your uncle, their policy covers their building. Your furniture, clothing and electronics are yours and are not insured by it. The relationship changes the conversation and not the contract.
A fire you cause is still pursued
If you cause a loss, the owner’s insurer pays the owner and may seek recovery from you, and it does so without regard to the family relationship. That is an uncomfortable scenario and it is exactly what the liability section of a renters policy prevents from becoming personal.
The owner may not be properly insured either
A relative renting to family frequently keeps a homeowners policy on a house they no longer occupy, which can fail at a claim. Raising it is awkward and it protects both of you, because an uninsured building helps nobody in the family.