You don't give any indication of where you live but regardless, the reality is that as long as your granddaughter is living with a natural parent who is willing and able to make decisions in her best interests that you have little or no chance of getting custody. Also, despite bio-Dad's absense over the years, he is not out of the picture until your granddaughter is 18 and would most likely be given preference over you if it was found that your daughter and Step-Dad were not fit parents.
My best advice is stay in the background, don't make waves and be emotionally available to your granddaughter as needed. If you create a conflict of this nature with your daughter (Mom), you will only end up alienating your daughter and she will respond by denying you access to your granddaughter.