Will my daughter go to her grandmother’s care upon the mother’s passing?

I just found out that my ex has terminal cancer. Paternity was established at birth and we lived together until summer of 2015, when we unfortunately split. Sole custody was granted to the mother in February of 2016, with visitation following state guidelines. The mother has been battling cancer for 6 months, and has now been listed as terminal. Mutual friends have told me that she created a will, and "willed" our daughter to the child's grandmother. I want her to be raised by me, as her biological father. I just need to know what I need to do prior to, and after she passes. I have no ill will towards the grandparents and would let them visit as often as they like. I just want the opportunity to raise her myself.