Romantic comedies are love stories that make you laugh. So You Can't Take It With You obviously qualifies. It's funny, both in a 1930's quick-bantered way and in a totally slapstick way. And, besides being funny, it has a love story. Tony and Alice are in love, they overcome difficulties, and they end up happily ever after. That's romance, folks.
The odd thing about You Can't Take It With You, though, is that it's a romantic comedy where the romance seems decidedly secondary. Yes, Jimmy Stewart was a biggish star at the time, and Tony and Alice do a lot of the work of propelling the plot forward over the fireworks and other hiccups. But their story (more central in the play) gets rather lost in all the back and forth about munitions buildings and Mr. Kirby finding his inner harmonica.
You could say that You Can't Take It With You is a romantic comedy with bonus business dealings, maybe. Or you could say that it's a double romantic comedy. Tony and Alice are the more traditional (but less important) couple. But the main romance is between Grandpa and Mr. Kirby. At first they're at loggerheads, as Mr. Kirby tries to buy Grandpa's house. Then, Grandpa gets mad at Kirby:
GRANDPA: You're poorer than any of these people you call scum, because I'll guarantee at least they've got some friends.
(A sure sign that romantic sparks are flying!)
And then, at the end, Mr. Kirby realizes that he should just chuck his career and go play harmonica with Grandpa. Tony and Alice decide to wed… but only because the main romantic plotline has been resolved. Before the lovers can get together, the two old dudes have to get together first. And while Capra may have a romance with his lovers, it's clear that he loves those two old coots even more.