some

I would like to have some brother or sister.
I would like to have a brother or a sister.
I would like to have some brothers or sisters.

Use some before plural countable nouns or uncountable nouns: ‘some brothers’, ‘some books’, ‘some water’.

Use a/an before a singular countable noun: ‘a brother’, ‘a sister’, ‘an uncle’.

Some can also come before a singular countable noun when we do not say exactly who or what we mean: ‘Some man called while you were out.’

See ANY (1)