Dear Sir,
No, your wife's cousin (her mother's sister's son) is not her relative for section 56.
So your wife's brother and sister can send money to your wife regularly. If her cousin send any money to her, it will be taxable in your wife's hand. It is advisable to prepare a gift deed Everytime you receive money to avoid any future tax complications.
Your wife has to disclose this gift in ITR as an exempt income.
Please feel free to call/ revert in case of any doubts
Thanks and Regards
Abhishek Dugar
CA CS B.Com