My answer is that Leia had no training & time was running out. I think Obi-Wan thought it would be too late to get Leia to the state she needed to be in to face Vader & overthrow the Emperor.
But the real reason is that the siblings thing is a bad retcon that created problems.
It was meant to be a love triangle. Thats why in the scene when Luke calls out to her she was able to hear him because deep down it was Luke who she truly loved, not because she was force sensitive.