How can you tell rob and tom were in the same family?

Since the story is written with third person narrator, Dad is the father to both Tom and Rob. If he was only the father to one of them, it would have been “Tom’s dad” or “Rob’s dad”.
I think the fact that Dad is spelled with a capital D, is because of the writing style, and that the fathers actual name is irrelevant.

The story is written in third person narration meaning Tom and Rob had the same father. Therefore they are part of the same family.

Advise instead of advice