Family relationships are defined at script_initialize_aristocracy. For each faction, there are 8 lords those being fathers, 8 lords being sons and 4 lords being lords with sisters. And at native, knight_x_y+12 always being son of knight_x_y (for y from 1 to 8 ). It's not random, the randomized things are brotherhoods between the 8 lords.
Do as you wish to script_initialize_aristocracy to customize the family relationships among the lords.