I'm sorry, I don't know much about what an appropriate HB would be at that stage. I think the fact that they heard one at all is a GREAT sign!! I'm sure if one of the other ladies here knows, they'll post.
In the early stages the fetal heart rate is approximately 80-85 bpm. However, by the 9th week the heart beat will be in the range of 155-195bpm. I looked this up and I hope it was helpful. Like bam said hearing it is a great sign, and your sisters babies isn't much lower than what they say. I wish her the best.
At 5 w and 5 days, it is very lucky to be able to see the heart flutter. With my dd I was 7 w 2 days at u/s and my Dr. told me she was only going to "try" to see it. We did, so that was great. I have read that early on the heart is just beginning to beat so the rate has not increased to the 100's yet. I hope that is the case with your sister. I wish her lots of luck!