Did the kronides know that Chiron was their step-brother?
No idea really, I don't remember if such a thing is ever touched upon. In Hyginus' Fabulae Philyra is transformed by Zeus into a tree (at her request) after she saw the kind of child she gave birth to, but there's no way to say for sure if Zeus was aware of her relation with Kronos and of the fact that Chiron is his brother. But then, Philyra in other sources still seems to be a person long after Chiron's birth, such as in Pindar's Pythian 4.
I think it's very possible that everyone knows Kronos is the father of Chiron, though interestingly he is most frequently called by his matronymic, unlike the sons of Kronos by Rhea for whom patronymics are almost invariably used. This could be an indication of the fact that unlike them he is an illegitimate child, I suppose, or maybe of the fact that few people actually know who his father is? Maybe Philyra would prefer to keep that detail secret as much as possible since she seems to be ashamed of the event.
