This movie was built around Standard Movie Script Template #3, but the likeability of the cast and the outrageousness of their particular situation make things work OK.
Jason Sudekis plays David, a scruffy low-level Denver marijuana dealer. (Note: given recent news, this movie is totally out of date.) He is content with a modest existence with no familial strings or commitments. He's a decent guy, though; unfortunately, that decency leads him into an altercation with some malicious thugs, who steal his weed and cash reserves. Which puts him in hot water with his supplier, "Brad Gurdlinger" (Ed Helms).
Fortunately, Brad offers David an out: just go on this little trip to Mexico and bring back a shipment. David's wary, because he looks like the kind of guy who would be smuggling pot back into the US. So he comes up with a scheme: gather up some co-conspirators who could plausibly act as his family members. There's Rose, a weary stripper with a heart of gold (Jennifer Anniston); Kenny, a neglected lad whose mother won't notice or care if he takes off for a few days (Will Poulter); and Casey, a hard-as-nails street urchin (Emma Roberts). Their relationships are wary and fractious. But they clean up well enough to outwardly resemble a respectable middle-class family.
And of course, it's one disaster after another from there. But do David and Rose eventually fall in love? Of course; this is an essential part of Standard Movie Script Template #3.
And, assuming you're OK with standard 2013 R-rated language and humor tropes, things are pretty amusing. (If you've ever wanted to hear Jennifer Aniston drop the f-bomb a large number of times, though, this is your go-to.)
My sister-in-law says they showed this movie on her flight up from North Carolina. Did it even make sense?