Hi, Rob.
I think they are using the expression to suggest that something unusual is about to come down, like frogs in the rain. If they are a dysfunctional family, then their getting together will probably bring trouble, which is what the 'raining frogs' suggests to me. It's idiomatic, but it's creative rather than a standard idiom, IMO. However, I like it and think I shall start using it. To me it sounds as if something faecal is about to hit the fan.
