× 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.
When we use some before a countable noun, the noun must be in the plural form. ‘There were some teachers who didn’t like the idea.’
Note that we can use some before a singular countable noun when we refer to a particular person or thing and we don’t specify which one. ‘Some idiot left the door open.’
→ See ANY 1