Why oh why do I let stuff like this get to me and why haven't I learned to just shut these types of conversations down?

Some backstory: I have an older sister who I've been estranged from for about 10 or so years. That was my choice after years and years of emotional turmoil with her. She's a loving person, but she's also a Borderline, with a history of substance and alcohol abuse. At the time I made the decision to disengage, I just couldn't take "it" anymore. And by it, I mean the highs and lows of my sister's many moods, her lies, and her (mostly) self-created crises. I didn't even know at the time that she also had substance abuse issues; I just knew that her behavior had gone from bad to much, much worse. I had no energy to deal with my own life, a life that was, itself, far from perfect. For those of you familiar with Borderlines, it's very difficult to draw and enforce boundaries with them, so I didn't see much of a middle ground. And so I ended our relationship, and began (and am still in) therapy to deal with, not just my sister, but the rest of my dysfunctional family and upbringing.

So, with that context, when my mother told me today that she "was concerned about my sister," I should have politely told her that while I care about my sister, I am not interested in knowing the details of what ails her. But I didn't have that foresight. My mom proceeded to tell me that my sister is going blind in one eye because of Hashimoto's Disease. Now, there is some truth to that in that my sister has Graves' Disease (as do I) and GD can cause significant eye issues, including blindness. Hashi's and Graves are very closely related diseases. But the other details my mom provided where so factually inaccurate and/or illogical, that it made me wonder if this was just another example of my sister's lies (she has a long history of lying about or exaggerating health problems). For instance, my mom had no answer for me when I asked whether my sister's endo had made a referral (and an emergency one at that) to an opthamologist or ocular surgeon. I can just picture it: At her last endo appointment, the doctor may have talked generally about thyroid eye disease and, from there, my sister suddenly believed that she, herself, is going blind. I've seen it way too many times with her. The last time she called me out of the blue a few years ago, it was to tell me (falsely) that she had lupus.

Suddenly, I was emotionally regressing back to 2001--to a time that was not particularly good or happy for me, and back to a state of extreme conflict over my sister. I know I'm in a better place now, but between the guilt I sometimes feel over disengaging from her and the intense anger she can still inspire, I feel kind of down. I know that Borderlines often lie without being conscious of the lie in the same way that a sane person is, but while I strive for compassion, it's not always easy.

Sigh. Thanks for "listening." I'm not looking for anything in response; I just needed to vent. In fact, if you're read this entire post, you deserve an enormous thanks!