Can A Finalized Adoption Be Contested?

Location: Illinois

I will keep this short and sweet.

Baby was born 04/29/24 to my sister, who has a history of substance use and mental health issues. A report was made (not by me) to DCFS shortly after the birth. My sister and her then boyfriend dodged DCFS for a short time, and then eventually, in mid-May 2024, left the baby with me and left the city. My sister made contact with me in November 2024 from another state, indicating her desire for me to adopt the baby. I got her to confirm this in writing.

My sister and her by then ex boyfriend (the baby's father) returned to the state in June of 2025 to attend adoption proceedings. the adoption was completely uncontested. during the proceedings my sister made comments about how glad she was to get it over with. the adoption was finalized 06/09/2025, at which point my daughter had been living with me for almost 13/14 months of her life. After the adoption was finalized, my sister disappeared entirely.

Fast forward to now. My sister (who is behaving in a non sober manner, which is not terribly relevant but somewhat, in my opinion) has begun dating a new man and has become convinced that I coerced her into the adoption, which again was completely uncontested, and she says that she is going to petition to have the decision reversed.

I can't seem to find any evidence that this is possible, but I want to have my bases covered. is this real? what can I do to protect my child from the possibility of being used as a pawn in this situation?