It sounds like it was a chemical. Even though it's an early m/c it can still be so emotionally difficult. I had one back in November and had a really tough time. I'm sure you already know this, but while it's devastating, it's also really good news in that it's evidence she has the ability to get conceive, fertilize and implant! Sooooo many things have to work together properly in order for those events to even happen. When you think about it, it's a miracle anybody gets pregnant!
Almost always, chemicals are just the body's way of recognizing very early that there was a genetic problem with this pregnancy -- and that doesn't mean that all others will be that way! Our RE always says that in a healthy person only about 1 out 4 eggs are genetically balanced. I'm sure her RE will take this all as good news. When we had our chemical, we moved on to a new RE (for other reasons) and it was a good sign to them because it proved we could at least get that far.
Likely, it's not the time to mention all that as she probably needs time to grieve her loss first. But when she's ready, there's some positive to see in this. Hugs to you and your sister!
but until the beta starts dropping consistently, there is always some hope. I've seen cases where the beta barely rises (or even dips) and then takes off increasing and everything ends up fine. So, you just never know.