The HCG doubling every two days or so (at the beginning, slower later) is an indicator of a viable pg. A single read is a good start when it's high like that. 800 for 4w is outstanding (see also
http://babymed.com/tools/pregnancy/hcg/Default.aspx - e.g. 800 at 16dpo and 1600 2 days later)
Studies I've read conclude that progesterone (P4) of >20 is near 100% viable pg and P4 of <4 is near 0% viability. That said, your sister's 9 is in the gray zone. For my losses, my P4 was around 8 or less. On the other hand my friend's P4 dropped(!) from 20 to 10 or so and she was told to expect a mc --- her energetic daughter is 6 yo today! All different ways.
I am of the camp that supplements
after a BFP won't prevent a mc. Again, all different ways.
With a vaginal u/s, a uterine pg is visible near 100% when HCG is 1000-1500. You're sister would know within a day or two for a *singleton*; a few days for multiples.
WARNING TRAUMATIC LOSS. I know this because in May I had an ectopic (tubal) pg, my P4 was low 6(!) and there was nothing in my uterus at HCG 1500 - we waited a full week longer to ensure I didn't have multiples. Despite a bumpy but favorable response to medicinal treatment, my tube ruptured anyhow and nearly killed me (massive hemorrhage, four units replaced). Thankfully I am alive but less one tube.
That nastiness aside, I am always hopeful for a viable pg - for myself and others. I'm am hoping your sister's next u/s is good news!!