Do zebra finches mate with their brothers and sisters?

i have two litters of zebra finches, total of 6, 2 female and 4 male. i have been finding eggs not knowing where there coming from. i have 2 open nest and just added a burrow (tunnel made of hay) and hung it at the highest peak. two (one male, one female) of them started immediately building their nest inside. i am now waiting to see if they are the culprits of the egg laying. the answer is yes, brother and sisters mate. you have the option of throwing or keeping the eggs, that's your prerogative. do you want more? i have looked into adopted them out with no success. and i would never just let them go, they would never survive outside of Australia.