Hi,
You really need to sit down and discuss the details of your moms financial situation. SSI should be cover all but a few dollars of your moms medications. If isn't then you need to find out why. There are many different programs thru Social Security...including a program called "extra help". This covers any co-pays for medical treatment or co-pays for medications. Perhaps your sister just doesn't know that these programs exist. You can find all kinds of information at www.medicare.gov Good luck to you and stop feel guilty!