If your sister is habitually lying and stealing, FX is assuming that she must be in some kind of emotional distress and/or has some kind of behavioral issue that needs to be dealt with by your mother in a new and different way. Since you are understandably concerned about this, FX recommends that you speak with your mother and urge her to get herself and your sister into therapy. It is important that your sister be seen by someone who can determine the underlying issues contributing to her lying and stealing, and who can work with her on those underlying issues. It is also important for your mother to meet with a therapist to get guidance and support in how to deal with your sister in a way that helps your sister’s behavior to improve, and that protects her and your grandmother from losing money and being taken advantage of.
If your sister refuses to participate in therapy, as she might, stress to your mother that it is important that your mother is still seen by someone. Just because your sister doesn’t want to participate in treatment and in making positive changes doesn’t mean your mother has to do the same. Regardless of what your sister decides, your mother needs to learn how not to enable your sister’s behavior and how to be helpful to her in whatever way she can.
If your mother needs help in finding a therapist she can log onto the US Department of Health and Human Services Substance Abuse and Mental Health Services Administration website for referrals in her area. She can also contact her insurance company to get a list of in-network mental health providers or check with her primary care physician or with your sister’s school social worker or psychologist to get a list of referrals in the area.