cockatiel sitting on eggs

by Rose
(NSW)

Hi there.I have a female cockatiel that has laid her first lot of eggs 3 in all.I have no other birds,but some times I mind my friends cocatiel which is a male.She laid her first egg 13 days ago.So now I dont know if the eggs are fertile or not.She laid the eggs in a little cardboard which she use to play in.How long should I let her sit on the egg and what do I do if they dont hatch.We love her to bits and dont want to upset her.Thankyou hope you can help with my question.