I'm so sorry for your sister's loss. She could be pg but also false pos's are more common than we think. She may have had a chemical pg. When was her period due? If the hpt was pos then the hcg would show up in her blood. She can always have her blood re drawn. Tell her good luck and best wishes. Take care!
How long was it between her BFP and the bloodwork?
There is definitely a chance she is still pg. I have seen a few posts on here of that happening and the ladies were indeed pg. I don't know what would make that happen, but she should continue to take care of herself and still see if she gets positive HPT's.