Firstly, no, please do not inbreed your cockatiels. This can cause birth defects and genetic problems.
Secondly, even if they weren't related, the female is way too young to be bred. I don't allow my girls to breed until they hit 18 months old, to give their bodies and reproductive systems time to grow, develop and mature enough to cope with reproduction.
I agree with shaenne, mating closely related birds doubles up on any faulty genes they may be carrying and that is why you see week birds who fail to thrive, birth defects, babies that die before they hatch etc. You want birds that are not related to produce strong healthy chicks. I also think a hen should be at least 18 months before breeding her or else you will risk her life to egg binding, and the parents may not be mature enough to understand that must look after the babies.