@kestrel
This was the exact position I was in for many, many years with my siblings, and it's a horribly lonely place to be. I am really sorry.
But I made my peace with it and eventually the evidence was just too much to ignore, and they each came to face the truth when they were pushed too far. Not trying to say that's bound to happen in your case, but who knows? And if not, you have still absolutely made the right choice. You're not alone and we're proud of you, the whole "blood is thicker than water" bullshit is one of the hardest manipulations to overcome.