Google "low income mental health treatment (your city) to see if there are any results for sliding scale mental health clinics in your area.
Disability benefits must be applied for through the Social Security Administration. Contact your local SSA office or go to
SSA.gov for information on the application process.
If you find a mental health provider for your sister, see if they have a social worker or case manager who can direct you with information about the steps of the process.
It is difficult to get disability for mental health only. I know some people who have, but it took years of denials and appeals. Generally, there must be a qualifying diagnosis and proof that the condition impairs the person despite attempts at treating and managing the condition.
As for low cost meds- depends on what is prescribed. Some meds may be on the $4 list at Walmart. Sometimes you can get coupons or discounts by visiting the website of the medication prescribed. Again, a social worker or case manager may be able to help you find resources. Sometimes nonprofit agencies may have assistance as well. Google "prescription medication assistance" and your city to see if any results come up.