"If we get gay marriage will the next step be brother and sister marriage?"
In australia its legal to marry your cousin, Auntie, Uncle, nephew, niece, but not a same-sex partner.
See the Australian Government Official Website:
http://www.ag.gov.au/www/agd/agd.nsf/Page/MarriageGetting_Married
"a person may marry their aunt or uncle, niece or nephew or ‘first’ cousin"
