Gee, if I was to ever talk to your sister, I'd be saying "whatever" a lot! I think she's at the point she doesn't even try to make up good lies any more. She's been lying for so long, she just does it out of habit...whether it's a "good" lie or something that's totally obvious a lie.
In another week, or so, she'll have some reason for why she had to leave...it will be all their fault.
Although I do think the brain does shrink from long term drug use, I think a part of it is also your sister's mental issues and her need to be right all the time, and her need to have things going HER way. Once a rehab/treatment center doesn't do what she wants, she's out of there.
Keep on doing what you're doing, which is being an awesome aunt to your neice!
Hugs and prayers!
Amy