So my dad's best friend just died and he got custody of his daughter (her mom is dead and she had no other family) so it isn't like she is my step sister because he didn't marry her mom and I was just wondering if anyone knew what her relation to me is

If she is part of the family then that is exactly what she is. That being said. She is now your sister.

If she is part of the family then that is exactly what she is. That being said. She is now your sister.

No. She isn't (legally, anyway) He got custody of her. He didn't adopt her. She is your father's ward.

There is no relationship, your dad did not adopt her, merely has custody