That's rather ambiguous and slightly dangling in antecedents. "Roughly two-thirds have EITHER been accused themselves, actually a relatively small number," [?] "OR more commonly, are accused of covering up the actions of one of their priests."
Does that mean 2/3 are merely accused? Or that, in total and combined, 2/3 are EITHER accused of molestation OR covering up? Those are different crimes. Considering that some accusations may be from schizo-affective psychos, and therefore untrue, is not included in this sentence. This needs clarification.
It means 2/3 have either committed a crime or are covering it up. The actual number who have molested is what they are referring to as a"relatively small number."
In other words, a relatively small number have committed abuse, but a large majority have covered up for someone who has.
SD