Hi Delta- this is a complicated situation- and also you have the right to protect your finances. You aren't obligated to support this family. As hard as it is to step back, you realized that your enabling wasn't helping their independence and it also took a financial toll on you.
Looking at the members involved- other than the 18 year old- whose potential is unknown, they are probably not capable of being self sufficient. It's good that they have the pension but are not capable of managing it.
An important part of this is who gets the pension. The elderly aunt may be eligible for some Medicaid care at home or a nursing home (it's different than Medicare). I am assuming you are in the US but if not, this might not apply.
However if she gets the pension, and it's over a certain ammount, she may not qualify. If she did, Medicaid would take the pension but this would leave the others in dire straits. So that's not a good idea for now.
If the 50 year old has an intellectual disability, she may be eligible for social security disability if she qualifies.
If whoever gets the pension is found to be mentally legally incompetent, a family member can make a case for financial guardianship if someone reliable is willing. This could incur legal costs and a court appearance and so would only be worth it if certified as incompetent by a medical professional.
I think a first step in all this is for you to make an appt at the social security office to discuss the situation and get ideas of what they may qualify. For the older person, a local council on aging can give you information, if any.
If the adults are legally competent, there's not a lot you can do to intervene. As social services once said to me "your parents are legally competent to make their own bad decisions".
It makes sense that these family members will be manipulative. Feeling guilty, I think it's a part of the situation because you do care but you also need to have your boundaries. You have no obligation to support them. For me, groups like CODA, ACA, helped me to manage my own tendencies to "help too much".
Sometimes people don't change until they have to. It may be that the whole situation has to topple before changes can happen. That change may be that the elder person eventually needs a nursing home, or they depend on resources like soup kitchens. If the living situation gets bad enough, social services may need to look into it.
The younger one seems to be trapped in this situation. However, she is being useful and gaining skills as a caregiver. One idea for her may be to start with getting her nursing assistant certification at a local community college, and then she could also work a bit while caring for her grandmother. With some maturity, she could continue schooling later if she chooses. She may first need to get a GED. It's also possible she has some intellectual disability if her mother does but if so, there may be supportive work programs for her.



