If the mother already moved and left the child behind and in the care of the grandmother, you should get custody of the child pending the outcome of the hearing. The fact that she placed the child temporarily in the grandmothers care rather than yours is certainly a factor the court will consider when making a decision. If she puts the child in the grandmothers care instead of yours, what can you expect if the child is allowed to move with mom? Keep in mind that the mother doesn't need permission to move, but she does need the courts permission to relocate the child. Review my contributions for relocation factors.
No one can say for certain if it will seal the deal but it is certainly a factor. The court should look at numerous issues before making a final decision. Good luck.
I have been a criminal attorney in New York for almost 25 years. website: Brooklynlaw.net Phone #: 718-208-6094 email: email@example.com. This answer is only for informational purposes and is not meant as legal advice.