Does she have a personality disorder by chance? Not trying to diagnose but something is just off. She seems unusually attached to people (you in this case) and is afraid to be alone? Or is she just lonely? If she works and raises a child then she must not have that much free time to be sad about not seeing her sister.

And something else comes to mind. Is she crying after she rolls her joint? Is it “high” crying? In my sad experience with people who use substances, they cry or feel other extreme emotions when intoxicated or tend to come up with weird ideas.

Overall just live your life and see your sister when and if you have the time.