It goes back to what is called ngeli in Kiswahili, which is the classification of nouns in singular and plural which will determine whether 'ya' or 'za' and other similar terms will be used. Za is used when the noun in question is plural, and ya is used when the noun is singular. In this case, kazi or work is assumed to refer to a lot of work that the brother has done, hence the use of "za" and not "ya".

In Swahili 'ndugu' doesn't necessarily mean brother. Ndugu roughly translates to relative or relatives. It's not specific to any person or sex.