I understand the problem and think an SQL code would exist (or could be written easily enough) but would exactly do you want your output to show? I imagine a table with all the males along the top and females down one side. The inner part of the table could be populated with a 1 or 0 given the couple is compatible?
Can step brothers and sisters mate?
I'm thinking a pivot table could work for this as well if you add some field(s) with a 'couple' id.