Is there an expiation upon a woman whose husband has passed away for shaking hands with her relatives (her paternal uncle and her maternal aunt's son) during her 'iddah period?

Shaking hands with a non-mahram woman is a reprehensible act, whether she is in her iddah or not. As for doing so from behind a barrier, there is a difference of opinion, but what is given fatwa by us is that it is forbidden. There is no expiation for it, but rather repentance.