You didn't apply for the visa. Your mother-in-law applied for her own visa, and no one can cancel except the Department of State or an officer of the Customs and Border Protection. If you submitted evidence in support of her application (such as an Affidavit of Support or an invitation letter) and you now regret it, you are welcome to inform the consulate that you are withdrawing whatever support you previously provided to her application. What, if anything, the consulate will do about it is completely up to it.
It is the applicant who applies for B-1/B-2 visas, not a third party. Therefore, a third party cannot cancel a B-1/B-2.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
You didn't apply, your mother in-law did. You can't cancel it.
While this answer is provided by a Florida Bar Certified Expert in Immigration and Nationality Law, it is for general information purposes only and an attorney/client relationship is neither intended nor created. You should seek out qualified counsel to review your case and provide you with advice specific to your situation. Call +1-561-478-5353 to schedule a consultation with Mr. Devore.