WMG / So Weird

Fi decided to stay with relatives to protect her family.

Realizing that being a "lighting rod" for the supernatural and attracting demons, aliens and vampires the rest of her family could end up suffering a similar fate to her father, so Fi decides to stay behind for they will come for her alone.

Also, Annie's supernatural magnetism isn't as strong or as potent as her own so while all the lesser bad spirits follow the family the badder ones are distracted.

The reason Death gave Fi's Grandpa more time was to make up for the time stolen from her Father

While Fi was certainly Badass and convincing when she stared down Death, Death is still a force of nature that typically does not bend to even the very reasonable begging of mere mortals...except Fi brought up how Death already took her Father, and Death admitted that wasn't it's doing, that her dad was taken before his time.Death is all about balance, it would make sense if it felt like there was a debt owed in this particular case.

It wasn't Rick's meddling with the supernatural that made the family a target for it, but instead it was the fact that Molly was a target for the supernatural that started Rick's interest in meddling with it

Over the course of the series we learn that Molly's family line descended from a particularly strong witch and was under the watch of a banshee, and a dropped story line showed that Jack would have been revealed to have been the reincarnation of a knight from that line. Besides that it was Molly who unknowingly made friends with an nearly immortal girl as a teenager (well before she even met Rick), and garnered the romantic attention of a merman.