How could Hera, Hestia and Artemis get to sire children? I mean, it's like a rule not to have babies with humans or anyone(for arty), right?

It's not a rule that they cant have children, they swore oaths to never have children, there's a difference.
for Hera, one of the other gods might have given their child to her as appeasement.
Same thing for Hestia.
As for artemis, they are hunters, not children of artemis. they join whenever they want if they're accepted by Artemis.

OHHHHHHH i get it! thanks!

THe reason is that they either want to stay true to their husband, or they want to be maiden forever, and not get smittened, and become stupid because of men.