My son’s name...
Just curious as to why “son’s” is rendered in the plural in Hindi, as in “My son’s name...”: मेरे बेटे का नाम... Why isn’t it just बेट का?
And also when brothers read books, why does the read become plural and not brothers?
The singular (son) is बेटा and the plural (sons) is बेटे.
However, nouns in Hindi tend to change form when they are in the oblique case. This happens when they are followed by a postposition (like का). The oblique case form of the singular बेटा is बेटे (incidentally the same as the direct case plural) and the oblique case form of the plural बेटे is बेटों.
So, मेरा बेटा छोटा है - My son is young मेरे बेटे छोटे हैं - My sons are young मेरे बेटे का कुत्ता - My son's dog मेरे बेटों का कुत्ता - My sons' dog
In 'मेरे भाई किताब पढ़ते हैं', brothers (भाई) is plural but it so happens that the plural of भाई is भाई itself. Think of it like the English word 'deer' which is its own plural.
For masculine nouns, only the ones ending with आ change form when pluralised (the आ changes to ए). The other masculine nouns remain the same as their singular form (so plural form of सेब is सेब). But of course, you have to use the correct verbs/adjectives etc if you're talking about the plural. So, it is nothing unique to 'their' or 'my'. They become उनके/मेरे when followed by a plural word (whether or not they look different from the singular form).