(1) This is not your child.
(2) Your sister (or brother per your post on the other board) did NOT fight tooth and nail to get their baby back. Reunification plans are usually very simple and they clearly have not completed theirs in the two years they have been given.
(3) This is a not a reunification situation where foster parents are fighting to keep a child from their parents. It is a situation where the only parents this toddler has known want to maintain her stable, loving environment and not have her sent to strangers who did not step up when she needed a home (regardless of the reasons).
OG said that in Ohio, you may have a chance. I have seen numerous cases in Illinois where a relative decided to step-up as TPR was happening. I never once saw the judge remove the child from the established home and to a relative that was a srtanger to the child. One grandma in particular stands out...she told the foster mom that she hadn't wanted to be bothered with all that baby stuff and diapers but now that the baby was older, she was ready to take him. How DCFS and the judge may rule in your niece's family court will depend on the culture of that court.
(4) Very often, the type of dysfunction that leads to a child entering care is systemic through the family. Caseworkers want to be sure that they are not placing the child into a similar situation that they were just rescued from. I have met some amazing relative caregivers, some ones who were barely adequate, and I saw one situation where the child was placed with a relative and then had to be removed from that home for abuse!
This baby is not returning home to her parents. This baby - emotionally - has a set of parents (fosters). You WILL traumatize this baby if you take her from her home. Some children are resilent, others never recover from it. I think you need to use a bit of Solomon's wisdom here...if you truly cared for your niece, you need to let her stay where she is.