Simultaneously horrifying and soothing. The family unit has always been a focal point for much of Shyamalan's filmography (take the ways in which the protagonists' families bind them to facades of normalcy in The Village and Unbreakable), but it's only here that Shyamalan explores the way in which familial hierarchies can quite literally wound those who are a part of it. The increasingly grotesque manner in which Old's characters fall victim to their surroundings reflects their anxieties- essentially, they fall victim to their own obsessions and obligations.
Parallels to The Happening are immediately apparent with the often disorienting manner in which this is shot (the slow pans away from characters almost seem like attempts to try and prevent their grisly ends by not entrapping them in the director's vision), but where the ambiguous nature of that apocalypse lets us hope for salvation, the physical boundaries of Old prevent any illusions of hope from coming into being.
It is no surprise, then, that in such circumstances, the bonds (and absence of such) between family come into sharper relief than in the realm of comfort. Catharsis- but at what cost? It takes entire lives to make peace with the fact that we may never quite understand each other, that for all the status occupations grant us, we are fundamentally fragile, fallible beings. The waves will wash over them eventually, so they might as well build a sandcastle; fleeting, but so are their lives.