Here are some thoughts:
1. You can issue a letter deferring her joining date to March or April 2015 as per your organization's convenience. This way, you can hire someone now who can join sooner, and at the same time, account for any attrition which will surely happen between now and March 2015.
2. There shouldn't be a problem withdrawing the offer, provided you can prove that the candidate did not disclose this required information at the time of the interview. There should be some declaration format where the candidate has signed off saying that all information shared is true and correct. Yes, it would be discrimination in a minor form; however, the organization is not a charity. Take legal counsel before going down this path.
3. In this situation you shared, she is not going to be entitled to ML anyway. So let her join, provide basic induction, team assignment, and let her work from home at 50% of the offered salary. That way, both the employee and the organization will gain.
Make sure you have taken all approvals in writing before you communicate with the employee.
All the best.
Regards,