how old is your brother? how old are your brothers? please explain is/ are

"Your/my/our/his/her/its/their" are all possessive adjective which are used to show possession related to anything or anyone. Which type of verb plural or singular use after it, it only depends on the given noun after it. As in your given sentence, in the 1st one, the noun(brother) after your is singular, so it's been used a singular verb "is". Whereas in the 2nd one, the noun(brothers) after your is plural, so it's been used a plural verb "are". I hope, you'll get understood it better after reading my this logic.

In the first sentence "your brother" is singular noun so we used in this sentence "is" and in the last sentence "your brothers" is plural noun so we used "are".

how old is your brother