This must be really hard for you. I'm so sorry.
I experienced something similar with SIL years ago. I lived with DB and SIL for a couple of years in our twenties and thought SIL and I were really good friends. One evening, I spent time with her "best" friend, who'd had a bit too much to drink and told me SIL never stopped complaining about me; I was a burden she put up with for DB's sake.
Luckily, this was pretty early on; your situation sounds much more upsetting because she's been able to keep it up for so long.
In retrospect, I don't think SIL always lied about her affection for me, and it's more apparent now that her criticisms mainly expressed her "victim" mentality. There's always an avoidance of accountability, and she'll frequently cut off her nose to spite her face if it allows her to point out how "put upon" she is.
For example, despite being wealthy, SIL won't pay for a private diagnosis for her child, but she will complain non-stop about how the health services, school, etc., are letting him down by putting him on a waiting list.
The last time I saw her, she slated her sister, whom I'm sure she loves. Her sister suggested they work together. Instead of saying no because obvious factors would make it difficult for them to do so, SIL agreed and then spent months complaining to friends and family about the detriment these factors were causing her.
The theme is always that SIL is perfect, but those around her are less so, making life very difficult for her. If only they were as kind/helpful/reasonable/insightful as she is, then she wouldn't be so put upon.
To get the feedback she wants, something like, "poor you, you're so kind/lovely/wonderful; you don't deserve to be treated this way," she needs to point out the deficits in her "aggressor". Sometimes she's subtle and tries to put people on the path to that conclusion by themselves. Other times she's more overt, especially if she feels her back's against the wall.
Does any of this sound familiar to you?