Everything in Shadowhunter lore taught them about the mortal instruments, and the powers they possessed. Very few books told them of the Keys, a powerful addition to the mortal instruments, incase the instruments were rendered useless. Only someone of mixed blood, vampire, warlock, seelie, werewolf and Nephilim blood were able to access the power of the keys. So they could not be used for harm against the Downword.
Once the previous Keeper of the Keys died a new child was chosen by fate, or some would say destiny.
Maryse Lightwood just happened to be carrying that new child, along with another.
Maryse and Robert decided to let Valentine experiment on the fetus twins, as he was wondering which one would end up with which blood type, but as the war was coming to a close, Maryse and Robert fled, forgetting entirely what Valentine had subjected their children too.
They soon remembered when Isabelle Lightwood and Elliot Lightwood came into this world, Izzy loud and screaming, Elliot barely breathing and silent. Maryse looked down at the baby and decided to send the experiment to Valentine, after all, she was only an experiment.