You use 'ka', 'ki' or 'ke' depending on the gender and number of the noun following it. 'ka' or masculine singular, 'ke' for masculine (or mixed) plural and 'ki' for feminine singular and plural.

So, जूलिया का भाई = Julia's brother and जूलिया के भाई= Julia's brothers (since भाई remains भाई when pluralised).