My family routinely overshares on FB :/.
In the instance you described, the woman must have really needed to unload and glad you were there to listen, even though it was uncomfortable. I can imagine you trying to find the right moment to exit but you are very kind and patient person.
I had someone reveal their despair about the holidays today because it enhanced the sadness they felt about losing several loved ones. It came bubbling out and she was near tears but the conversation seemed to help both of us and the other person there. Gave some perspective and a reminder that people are going through lots of emotional upheaval as we pass them going about our own business.
While I didn't know the depth of her sadness because our situations were different, it was nice to get beyond the false cheer that sometimes rules during this time of year and connect with someone. People feel that have to put up a front and she let down her guard today and had two people willing to really listen. Sometimes that is all a person needs at the moment.
Backing up to the part of your post where your sis wore your present before sending it in July, it made me pause, LOL.