As a Muslim man, is it permissible for me to assist my adult adopted sister with washing while she is unclothed, if no female caregivers are available and the care is necessary?
Your adopted sister is not your real sister, so, you are not allowed to look at her unclothed. Hijab rules apply on you and her .
Emergency has got its own exceptions just like emergency rules to save life of any other female who is non Mahram to you.
Wassalam.