The money stayed by the beach for 6 months before Jack retrieved it. Right away that tipped us off something wasn't quite right. Why would they leave their family fortune far from the house and in the open like that.
Also, while it's established that Allie was a friend to the 4 siblings, she only saw Jack for 6 months and never visited the house or asked to see the others? That seems very unlikely.
I figured, she was mainly close to Jack, and she was just friendly to the siblings by association. I don't think I would have bat an eye if I didn't see a close friend's younger siblings for a few months.
And with everything going the way it was, it was clear he had his reasons to keep her away from the house. I presumed she'd asked (I thought that even came up in the movie), but he'd just say no because of his mom.
I don't think six months would have been enough for her to be extra suspicious or anything.