"I have two grandpas with sisu."
Translation:Minulla on kaksi sisukasta vaaria.
The hints don't take context into consideration, so the hint is actually correct because "grandpas" is plural and "vaarit" is also plural, whereas "vaaria" is actually partitive singular. The reason why it's singular is because numerals higher than one trigger partitive singular. A plural is used with a numeral only when the numeral itself is plural (for example "kahdet" instead of "kaksi"), i.e. when there are several sets of something.