She has no relationship with your step-daughter. They do not share a common ancestor. If you adopt her, they would become second cousins once removed.
Second cousins
Your niece and your daughter are first cousins. Your niece's daughter and your daughter are first cousins, once removed.
Your niece and your daughter are first cousins. Your daughter and the daughter of your niece are first Cousins, once removed.
The children of siblings are first cousins, so your daughter and your brother's daughter are first cousins to each other.
Your cousin's daughter is your first cousin once removed. Your daughter and your cousin's daughter would be second cousins.
Your daughter and your niece are first cousins. Your daughter than your niece's son are first cousin's once removed. Your daughter's child and your niece's child are second cousins to each other.
Your daughter and your nephew are first cousins to each other. Your daughter and your nephew's son are first cousins, once removed, to each other.
Your sister's daughter and your son are first cousins to each other. Your sister's daughter and your son's daughter are first cousins, once removed to each other. Your son's daughter and your sister's granddaughter are second cousins to each other.
Your daughter and your first cousin are first cousins, once removed.
Your daughter and your first cousin's son are second cousins to each other.
Your daughter and your second cousin are second cousins once removed to each other.