Sandra had been told, many times, of how serious and upsetting OP's condition was. She continued to completely disregard this (e.g. "so are you still up for the party?"). This is not a momentary lapse of memory or a brain fart. She has done this several times, and has been told the facts of OP's conditions several times. This means her behaviour is no longer excusable as someone who is clueless and doesn't no any better--she has been told otherwise, and cannot be said to be hurting the OP due to cluelessness any longer. Maybe she's a "nice person" only some of the time? Because that's a great way to tell if someone isn't actually a nice person at all.
That said, there are two sides to every story, and although I cannot personally fathom what could excuse Sandra's rude, inconsiderate, and downright dangerous behaviour, there might be some reason. Everything she has said and done screams "uncaring and selfish", but whether or not this is the truth cannot be said. In the end, though, it's irrelevant.
Whether or not Sandra is a good friend, her actions are nevertheless taxing for Nemesis, who is not in a position to endure it easily. If the cause cannot be treated, treat the symptom. I honestly think it would be best for Nemesis, who needs as much time, rest, and mental strength as possible at the moment, to completely minimise contact with Sandra, or block her entirely, since Sandra has confirmed that her actions are only going to make a bad situation more stressful and worse for OP. Doing what is necessary, for your own safety or health, is never something you should feel bad about or hesitate over.
Many many hugs to Nemesis, and lots of wishes for strength.