I wouldn't allow any more sleepovers. Work something out to ensure this. Up to and including showing up before bedtime to retrieve your child. On another note, is the 3 yr old capable of understanding that she is causing harm? If the mother is not able to discipline her children, that's one thing, but if this toddler is not able to control her actions through a dissability, that's another. If it's the latter, that's another reason for an intervention from child protective services. I would suggest hiring legal representation. I know you mentioned it would take 2 yrs., but it seems there may be problems down the line and this is the tip of the ice berg. The last thing you want to say is, "I should have taken action when we first started a problem with the biting issue." If by hiring a lawyer, it jars the mother into more hands on parenting, it is a win win situation. Doing nothing and expecting something different just seems feasible, especially with the health and well being of a child in the balance. Acting out on your child's behalf, is more important in showing her that you will exhaust every means necessary to keep her safe, I can't imagine she would feel as loved or cherished if her parents just threw their hands up in the air citing, impossible avenues as an excuse.
I have a son with a dissability and I am a stepmother to my 27 yr old daughter. I thank God everyday that he gave me the courage and determination to speak out against injustices against my son and get him the help he needed. He is now a normal 17 yr old football player. Just a neat kid. I also met the most precious angelic child in the world when she was 5. I married her father. Her Mom, my husband and I have always done what was in her best interest and I'm grateful her Mom shared her daughter with me. She is my daughter. She simply says, "I have two Moms!" I just hosted a beautiful baby shower for her and am now anticipating a grand daughter in November. I wish you the same successes with your family. God Bless you all.