No, that's right as well! When addressing someone who's older, you use the formal way of addressing someone, which in Hindi means you use the plural form. So उनका / उनकी / उनके are the words you use for plural as well as anyone you wish to convey respect to, such as elders.
If it were 'his' or 'her', it would be the particularly formal/respectful form to use उनका, in this case, it might be quite unusual to speak so formally/respectfully of a person, and not to afford the same to their brother - viz. it is not के [...] जाते हैं।