Thank you all for your excellent responses. I don't have access to most JW literature, so the quoted articles were especially helpful in seeing that she did step well beyond what would be allowed had anyone she knew seen her.
Xander, you asked if my sister commented on our services afterwards, and yes, she did. Also, during the meeting she bowed her head and closed her eyes in prayer, effectively becoming part of the service with us. She listened intently to the speakers, and afterwards commented on how beautifully one speaker in particular, a teenager, prepared and presented his talk and what excellent points he made. She wanted to know if he had an outline (he didn't...we never use them).
She sang the hymns with us, commented on how well the organist played, and when one of the sisters sang a solo, my sister dabbed tears from her eyes.
I barely dared breathe during the entire service, expecting at any moment that she would walk out or at least close her eyes and pretend to sleep as I'd seen her do before when confronted with non-JW doctrines or practices in situations where she felt she couldn't just walk out.
I didn't comment on her own religion's stand regarding her being with us as I really wasn't sure if I'd misunderstood what that was, or perhaps it had changed. If she doesn't know that I know what the teaching is -- thanks to all of you -- perhaps she'll attend with us again and the armor's crack will open a little wider. I love my sister and will support her in whatever she chooses to do, but I know her religion has not been a blessing to her other than to give her a never before had sense of belonging. She's given up so much to gain so little.
Thanks again...I really appreciate the insight all of your comments have given me.
Susan