Hmm.
I broke my 4th rib right at the sternum. I knew immediately when it happened because I felt like I'd been kicked in the chest.
However, I have brittle bones and I was simply getting up from a laying down position when it happened. The "pop" was audible. My son in another room asked "what the hell was that?"
I couldn't speak. I couldn't even catch my breath.
I thought maybe I popped something out of whack.
It happened on a Saturday and I suffered in agony until I went to work at my hospital on Monday when they got me in for x-rays.
There was no "trauma" involved and I couldn't believe I broke my rib. But I did.
All I can think is that if your sister keeps herself so medicated, she may not have been able to feel the pain when it happened. I certainly did. How she got the injury is anyone's guess.
Breaking anything in the chest is extremeley painful. That's all I know.
I managed to do it without wrestling, being in a car accident, falling, being hit in the chest. I knew the second something was wrong. But, like I said, I didn't suspect breaking anything because all I was doing was getting up.
I'm waiting for insurance authorization for a bone scan because I tend to break easily.
If you don't have access to her protected medical information, you may never know if she is lying. Or exaggerating. Or brittle enough to break something.
The upside is that she sees a specialist regularly so hopefully they are tracking all these things.
She may be having more issues than you know.
Best wishes.