Who can contest?

Here is our situation: We were picked by the children's parents and social workers to be these children's parents. They are 3,4 and 18m. Currently they are living in their emergency foster placement (grandma and aunt) Parents go to court and sign consent. Judge tells court to no longer look for relative placement and thanks the parents for making this process easier on the children. After court, step-sister to grandma claims to have hired a lawyer and is planning on contesting. Who can contest an adoption? The parents are signing TPR on the 20th and are standing their ground that they don't want any relatives to have placement. They have said that if the children get out of the family then they will have a chance. Really these parents love their children they just can't raise them. So my question is who can contest and how long can this be an issue? We are in MN if that helps.