What can I do legally to protect my sister from our mom?

Please Excuse the lengthiness of this.

In 2008 my husband and I took custody of my sister from foster. Her mother (mine too) had left her with friends and never came back till she found out that my husband and I were going for custody. She had many chances to be in her life back then but she never took the state up on supervised visits. She was found by the state to be guilty of medical neglect and custody was given to us until she was an adult. Her mother was not allowed to communicate with her via email or social media as she would tell her lies that she would begin to believe. Anyone who has been a foster parent I'm sure can relate. During the time of her being a minor the state helped up to get SSID setup for her as she has CHARGE syndrome and seizures as well. I was appointed the payee and still am until she's ready to move out on her own. She's now 21 and we are working up to her moving out on her own in May if we can find suitable housing and assistance for her. Up until last night I felt secure that she would be safe and could go on to live her life on her own with limited assistance. Last night I found out that her mother has been contacting her via social media and talking to her about really personal items. A couple that has me worried and I'm not sure where to turn or what to do. I feel like she's in danger and that it's my job to protect her from being taken advantage of. Her medication that she takes keeps her stable and is needed. Her mother was directing her that she needs to go off of her medication. After that she goes on to inquire about if she's in control of her finances and if not how to go about going about doing so. I'm not sure if there is anything that I can do but I thought I'd ask for advice. I know my mother and she'll take advantage of her the 1st chance she gets. She has a history of using people in our family for money ,who are not mentally competent to tell her no. Any advice on what I can do legally would be appreciated .