What happens if I have 2 brothers
Siri will ask for clarification.
Also, you could change your relationship with each of them to custom labels e.g. "Older/Younger Brother" like so, in addition to the default "Brother":
This way if you ask non specifically to contact your brother Siri will ask for conformation 'Name 1, or Name 2?', but you may be more specific from the start.
I'd hope that by the time Siri is out of Beta Siri will be able to interpret synonyms of words, so texting your 'little brother' is associated with "Younger Brother", who is in this case 'Name 2'.
Similarly, if Facebook is integrated into future versions of iOS the user could ask to call or otherwise contact a relation of a person in their contacts (but who they don't have in their contacts) - So asking for "Tim's cousin" finds "Tim x" in your contacts, then determines who Tim's cousin is by looking through his profile, picks out the cousins phone number from their personal info page (assuming their security is in this regard set to 'friends of friends' or lower) and initiates the call.