I have a friend on this forum who also purchased a half sister to mine, and the chances of her having it, is very slim. But I can still understand the concern and I am not worried.
The thing is, it really isn't slim. It's a roll of the dice. Unless both parents were HYPP n/n then she will be HYPP n/n which is fine, but if either parent (or both parents) are HYPP n/h or HYPP h/h she stands a chance of having it also.
Something I just found about HYPP that might help:
"Myths about HYPP
Some people have felt that the disease can be diluted out and not carried to distant generations. This is false because an affected horse has just as much chance to pass on the trait as the affected parent which passed the gene to him. Some people also believe the horse will "grow out of it." This is not true. For unknown reasons, attacks of HYPP tend to occur most often at the beginning of intense training and fitting for shows (age three to seven years old). It is important to realize that horses with HYPP are affected for life. It is possible that older horses do not experience the same conditioning stresses as young horses or owners have discovered the best management strategies for the older horses with HYPP.
Some people also think that if a horse does not show any signs up to a certain age, it does not carry the trait. Unfortunately, this is not the case. Once again, horses with HYPP are affected for life. There was a stallion and a broodmare with HYPP who did not show signs of the disease until age eight and 15, and both horses only experienced one isolated attack.
Owners and breeders of affected horses should inform prospective buyers of the management constraints these horses have and the potential for future episodes of HYPP."
HYPP could happen while on a ride, the horse could just fall out from under you. In severe cases of HYPP the horses basically suffocate to death.
You can have the test done yourself here for only 40.00: http://www.vgl.ucdavis.edu/services/horse.php
all they need are 20-30 hairs with roots to test, put in envelope with check and mail to them.
Better to spend 40.00 now, than the heartbreak of HYPP and the maintenance and expense later.