I need to think it through a bit more, but my first thought is for each SS to send her receiving sis a 'little something' to wear to the party. Then she also wears the same thing. For example, a yellow ribbon, or a SS badge, or stripy socks or whatever. Then when they arrive, they look for the person with the same thing.
Alternatively, have each SS send her receiving one half of a pair an d they match up on the day. eg, one earring, or again socks, gloves (even the rubber variety!) On arrival they have to find their other half.
I'll think a bit more about this one.
Every day we write a memory on the heart of our family. What memories did you write today?