Horiph
Zebra finches, will begin to mate and lay at amazingly young ages.
The fact that they are siblings will not stop them from breeding . Siblings will mate with each other, as well as parents with their own children, grandchildren, etc.
If you have a nest in there, I would remove it promptly, to discourage further breeding and egg laying. Your girl may still lay an egg here and there, but without a nest, hopefully will not raise any young.
Make sure that your girl has enough sources of cacium available to her (such as cuttle bone, and crushed egg shells).