I think Divine hits on a good point, Sarah might not be all that 'great' but your friend may just click with her, friendship chemistry isn't always straight forward.

I too wonder why you feel threatened by her. It's not unusual not to like all of a friend's other friends - we have different facets to our personality and different friends fit that, but it seems to trouble you to the point you want Sarah gone which is a strong reaction. This might be hitting on some nerve within yourself.