In general a parent has a right to raise their child as they see fit... which includes allowing them or not allowing them to see other family members. That said, New York does have a mechanism for allowing grandparents to seek visitation.
Section 72(1) of the Domestic Relations Law states that
"...where circumstances show that conditions exist which equity would see fit to intervene, a grandparent may apply to [supreme or family court] and . . . the court, by order after due notice to the parent or any other person or party having the care, custody, and control of such child, to be given in such manner as the court shall prescribe, may make such directions as the best interest of the child may require, for visitation rights for such grandparent or grandparents in respect to such child."
This basically means that the grandparent has to show the "equity" requires that they be heard on the issue. Then, if the court hears them, the grandparent has to show that the best interests of the child are served by granting visitation.
As a sibling, there is nothing in the law that would give you visitation specifically. BUt you could file under the same statute, stating that equity requires it. The child's grandmother can definitely seek visitation though.
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