BILL HUGHES: What happened? You were all right last time I saw you.
GRANDPA VANDERHOFF: One of my granddaughters dared me to slide down the banister.
BLAKELY: Too bad, is it serious?
GRANDPA VANDERHOFF: No, just a sprain or something. The thing I like most about it is the crutches, I've been wanting to walk on them ever since I was a kid. Haven't you?
Grandpa hurt his ankle because he was playing with Alice. Family can get you in trouble—but also lead you to happy surprises, like Grandpa getting his crutches. In this film, at least, family never leads you astray.
GRANDPA VANDERHOF: Right up to the very last, she couldn't walk into a room without my heart going thump, thump, thump.
Grandpa's relationship with his departed wife ties together the themes of romantic love and family in the story. Grandpa keeps the house because it's where Grandma lived, so even though she's gone, their romantic love provides the foundation for the family. And that love is mirrored in Alice and Tony (Alice is supposed to look just like Grandma, according to Grandpa.)
GRANDPA VANDERHOFF: Do you have any Russian stamps for me?
KOLENKHOV: No, nobody writes to me anymore. They are all dead.
Kolenkhov's family back in Russia is all dead—but he's got a new family with Grandpa and the Sycamores. Again, family is defined by who you love and/or hang out with, rather than by where and to whom you were born.
TONY: I want them to meet you as you are, not some trumped up evening with everybody acting unnaturally.
Tony is more than a little in love with Alice's family, presumably because they're so much more fun to be around than the grumpy Mr. Kirby. He seems to have hoped that they would win over his own family—though it didn't really work out.
MR. KIRBY: Mr. Vanderhof, you once told me I was a failure as a father.
GRANDPA VANDERHOF: Oh, I didn't mean that.
MR. KIRBY: I know, but I am.
It's ultimately family that transforms Mr. Kirby— both Grandpa's family, and his concern about his own son Tony. Family, at least in I Can't Take It With You, makes you a better person.