"Yes, it is."
The non-gendered pronoun 'it' refers back to the other non-gendered pronoun 'this'. Once we have established that the subject is your brother, we can then continue with gendered pronouns. For example:
Is this your brother?
Yes, it is. He's my twin, in fact.
Is that your sister?
Yes, it is. She's come to visit for the weekend.
Who's on the phone?
It's my dad. He calls me every day.