The reason why you can't understand this sentence is simple - it has no clear meaning.
My guess is that it was written by a non-native speaker in an effort to use the use the expression 'rare as hen's teeth'. They are trying so hard to 'shoe-horn' this expression into a sentence that they have written something that just doesn't make sense. Perhaps they are trying to use the construction 'xxx was so xxx that....' (as in 'There was so little evidence that ....) , but the word 'that' on its own can't make the sentence work.
What the writer wants to say is that there is too little evidence to arrest her. 'Rare' is the wrong word here, and the use of the the hen's teeth idiom makes no sense grammatically.
Please forget about this sentence, Jenny. I suspect that your English is better than the writer's.