I have read this whole thread this morning with growing horror. At first I though you were BU but with the rest of the information you have given I have changed my mind. I still think you are BU but for a different reason. I think that, though meaning well, you are enabling your DSIS to manipulate everyone around her. She is using her illness as a stick to beat you all with and by continuing to facilitate this you are preventing her from having to face up to the true consequences of her disability, whatever they may be. And without admitting this no one can get out of this dreadful situation you are all placed in.
I have not commented before, as I have no experience of epilepsy or the other health issues you are faced with. BUT, what I do have is experience of is my DD, with chronic asthma and various other issues, including mental health. She used her illness (es) over the years, for attention seeking, excuses and nearly drove me to a breakdown. She lied to us, some of her illness was fictitious, some not, but we never knew which. She convinced specialist consultants to perform one, maybe two operations- they can do scans all they like but if a patient is telling them something and they cannot confirm it either way, and they have enough suspicion, they operate-at least in our case they did. It is the same, but opposite of what your DSIs is doing, by not giving enough, or the true information. The doctors, to an extent, go on what they are being told.
And because she was my child, I believed her, I fought for her. I sat in more hospitals than I can remember with her crying and swearing she was in pain. I martyred myself and neglected other DCs and my DH, who along with friends, one by one, shook their heads and walked away from her. Everyone could see it but me. Her lies and manipulation got bigger and bigger to control me, and in the end, when she was caught out, I was devastated. Now I'm not suggesting your DSIS illness is wholly fictitious or that she is the same as my DD, DDs primary issues were real, but the stuff that surrounded it was psychological. Separating the two is difficult, as they are both "real" issues.
I had to, for her sake and the rest of my family, step back, and allow her to hit rock bottom. We have the luxury of distance now too, so I cannot, and do not, go running. She is pregnant, having to inject herself every day (lupus something) my heart drops when she phones as I dread it being another problem-I'm not even sure if the lupus thing is true! I fear, when the baby, my grandchild, is born that she will use it and project her issues through it. I pray with all my heart that she won't and that it will be the making of her. I don't know which it will be.
What I am trying to say, is I do understand how hard it is and probably how scared you are. It is like crying wolf. She may be ok left once or twice but what if, once, it is the big seizure, that's serious, and there's no one to help. She is an adult and while you all run around her, she will continue to live in denial about how serious her condition is. This is unrealistic and dangerous. If she is well when she feels like it, but ill when others are, then there is more going on than epilepsy and without forcing her to take steps to at least get a proper diagnosis (i kniw the epilepsy is diagnosed) and treatment she is putting everyone under a great deal of stress and herself, and maybe a baby, in danger. You have to stop this cycle. Go home and care for your own children, and for yourself, because you are not helping your DSIS or anyone else at the moment. I may sound harsh, I have become so, but there are too many likenesses to my DDs behaviour for me not to comment.