He is heir __________ his grandfather prosperity.

وہ وارث ہے __________ اس کے دادا خوشحالی

for

He is heir to his grandfather's prosperity.

وہ اپنے دادا کی خوشحالی کا وارث ہے۔

You’ll often use *to* when you want to indicate a relationship between words such as possession, attachment, or addition.

Here "he* is attached to his "grandfather's prosperity".