hi sara
just wondering did your sis's doc request a preg test where the result is positive / negative or a quantitative BHCG where anything under 25 is considered neg. (but could be going up/down).

if it is still early it is possible the hcg is <25 and on the way up. (also the reason one HPT was neg if a different brand and less sensitive, doctors room tests are not sensitive at all and are generally for >50 hcg.). also if the neg result was from a sample other than FMU it may give a false neg.

i have heard that BT are sometimes wrong but with the modern tests this is highly unlikely.
your sis could ring the doc and have the request changed to a quant hcg that may give you some answers, otherwise i would retest over the next few days if AF doesn't come. and as they say while AF stays away you are in with a chance !

for your sis, that it is only a slow starter, and not an early loss.