They are not your Mahram nor your cousins, even if they were Muslims. Therefore, you must not shake hands with them.

My family is not muslim. Is it permissible to shake hands with my cousins’ sons? They are in their 20s up to late 50s in age.

Mahram | November 5, 2014 5:20 pm
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I fully understand that I am not allowed to shake a strange woman’s hand when greeting her as I am not a mahram to them. For the avoidance of doubt, please could you confirm if I can shake hands (or touch) with the following women:- my non-maternal/paternal aunts (ie. my uncles wives), my female cousins.
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The husband of my aunt is not a real uncle for me. What is the ruling for me with him?