Frank Capra was big into community. None of that "eccentric artist in splendid isolation" nonsense for him. If he's going to have eccentric artists anywhere near him, they're all going to live together in one happy family, with traditional marriages for all. Capra loves democracy and America and Americans, and so his films are always stuffed with crowds of people bustling and democratizing together.
Since Capra is so into groups and gaggles of guys and gals, it's no wonder that his narrative isn't so much "a narrative" as it is "a bunch of narratives." There's no one single protagonist wandering through You Can't Take It With You. Instead, the film follows a bunch of different stories—Grandpa, Tony, Alice, Mr. Kirby, and even bit players like Mrs. Kirby and Poppins get to share the story for a bit.
You might think you'd be confused with all those characters, but the narrative is simple and slow. When there's a point that needs to be explained, it gets explained repeatedly and with care. (At the end, for example, neighbors on the street run around telling each other that they've lost their houses over and over for about a minute. Capra doesn't want you to miss that important point.)
Overall, the narrative works less to rush forward, and more to tie everyone together. The plot is just there to introduce you to all these lovely people and get them together for the big celebratory happy meal and happy ending.