[FirstAndFinest]

I believe that being direct doesn't have to mean starting an argument. You can very politely ask her for your shirts/cups back - without offering a reason! They are yours, afterall! If she should ask Why?, then tell her that they are more than just silkscreen ink on cloth, that the letters have a deeper meaning that is known only to initiated members, so the shirts mean a great deal to you and can you please have them back. (might be a good time to request that she not "borrow" them again...)

As for the cups - that goes beyond just a deeper meaning. Tell her that our sorority does not approve of members drinking in letters and that drinking beer or booze out of a cup with letters is a blaspheme. (I am exaggerating but only slightly...)

Worst case scenario: lock your letters (shirts & cups) in your room.

Have you considered bringing this woman to a COR event? Maybe she wants to wear that diamond, too?