It sounds weird, I know, but... cancer's really interesting, isn't it? Scary and miserable as it's been, I have been left with such respect for the human body. I had an extravasation, and there was concern it could need plastics intervention, but it healed, even during chemo. And watching that happen so visibly (and the way the DIEP functioned, right from the first night) really made me appreciate how amazing our bodies actually are. In the past, I'd seen it as how they looked, and not how they're constructed, and their ability to heal and recover. I'm being put on the NATALEE trial post-oophorectomy, and the nerd in me is quite excited by that, too. I'm interested to find out, eventually, where the double-blinding puts me.
And having cancer obviously is random, and affects arses and angels alike. I met a woman in rads who had a small area of DCIS removed via lumpectomy. She started saying she couldn't have stood to have had her breast removed, or chemo because the hair loss would also have made her feel less of a woman. She was crying. I was just sitting there thinking, can you fucking find a more appropriate person to vent this at, please?! I ended up saying, "well, I have young kids, so I have other things to focus on, I suppose." "Oh," she said, "My son's twenty so he can look after himself." 
On paper, me and my 8cm tumour plus affected margins is as early stage as her presentation, when all she had were some non-invasive cells, swiftly removed, because in both cases, treatment has curative intent. And people don't understand that. They very understandably hear 'early stage' and assume it's a small issue. As a doctor, obviously you know the variations within that descriptor, but I don't think most people do.
As the woman I met demonstrates, having cancer doesn't automatically make someone worthy of unlimited sympathy!
For what it's worth, however ungrateful she was she is really, really lucky to have you. I heard some awful stories about single mothers having to cope alone. One with baby twins, and another with a child who had to be placed in foster care for a while as he had profound disabilities and she had no family able to step in. Your SIL, like me, is exceptionally lucky in having people rally round.
I appreciate all you did for her, even if she doesn't!