It just makes me sad to see her isolate herself from others. I've posted about this before, so this isn't a new issue.
How does she feel about it? Is she sad to be isolated? Is she happy with her choice or is something (anxiety/sensory/lack of common interests/social skills/???) holding her back from jumping in or participating when she wants to?
I ask because to my extroverted sister took decades to understand my not always wanting to be completely surrounded by people. To my introverted self however, everything was great. 1-2 close friends is perfect in my world and staying home with a good book was sometimes what I'd pick over a party which boggled her mind. Conversely when I tried to keep up with her hectic social life I eventually wanted to be at home alone after a week or so.
