At least now you know that his sister, while probably well-intentioned, can't be relied on or trusted. I had a very similar issue with my ex's sister - she seemed well-intentioned and was a help for a while (sort of...), but in the end she was still very much enmeshed in the family dysfunction, which ended up causing more drama and trouble for me.
This is spot-on. And after interacting with her this weekend, I am getting whiffs that she might herself have a drinking problem. So.