I have a sister-in-law who is on disability (not sure what it is) but she's not understandable when talking. 3 years ago she worked and when talking she was understandable. When I ask her what the doctor says she says she's not been due to she doesn't have a car ride now or the money. She's on Medicare (the one you don't pay she says). I believe she might have had a stroke (I myself am a stroke surviorior).
If your sister-in-law had a stroke, she needs to see a doctor immediately, even if the stroke was months ago. That way the doctor could get her into rehab to help her deal better with life, plus have some speech therapy.