Sure, I don't think Rhianna hasn't teached her or teached her bad. But is it different if you do it only to fulfil what you was ordered to or if you put your heart into it. The knowledge would be the same, yet I assume it will feel quiet different for the pupil.
And I give you the Azera question right back to you.
If Azera was so unfit to be the next queen then why is she the next queen? If her power and knowlede was so limited why would Anaiel chose her to be the next queen ? Or could even the present queen not change this? Both sisters would most likely be similar educated and as first and second in the succesion to the throne this would be done careful.
As for the power, I think, Rhiannas has much power as well Azera. It could be that Azera is power are more useful for the direct approach and Rhianna is more gifted for the work as an oracle. Or it simple could be Rhianna was shaped to be the next oracle and Azera not.