Is this a joint tenancy with a right of survivorship?

Absent facts you haven't disclosed, most of which would prevent your mother from reconveying the land (e.g., she's not mentally competent), your mother is not going to be able to undo the prior deed. If she wishes to do so, she should consult a real estate lawyer.

The easiest thing for your mother to do, assuming her assets are sufficient, would be to set up an estate plan which conveys other property to the other daughters, in lieu of the share of the real estate she is unable to convey equally.