We've been doing secret swaps for Valentine's and Easter. Maybe St Pattys was in there too, cannot remember.
I know there is talk of a Secret Santa coming up....still in the work stages on that....have to decide on dates.
I'm not really sure a Secret Sister would work. There are those in Canada and Europe that would be found out immediately just by the postmark. Also, if you send overseas, then you also have the Customs Form that needs to be filled out, so that "outs" the person there.
Just something to think about.