Wow - I knew there were lots I was missing, but I didn’t realize quite how many. Thanks, all!
My bad-- won best director for Leo McCarey. I knew it won one-- it was nominated for most of the top awards (Irene Dunne was nominated for best actress a heart-breaking 5 times, and never won), but only won one, I just got the wrong one.
Do monsters count? Sully becomes a foster dad (more or less) accidentally in Monsters, Inc. It’s only temporary.
Could the Star Wars series be a contender? After all,
Anakin/Vader was brought back to the light side by an opportunity to actually act like a father. It was the only part of his humanity that the Emperor couldn’t destroy. “Tell your sister- you were right.”
In Douglas Adams’ Mostly Harmless, Arthur Dent abruptly finds that he’s the father of a moody teenage girl, when the girl’s mother suddenly stops by and drops her off with him.
I think there’s some redundancy in the phrase “moody teenage girl,” but let it be.
Based on a true story, for what it’s worth.
Not quite “suddenly became parents” given that each of the two adults had a large number of kids to start with (then as I recall they had a couple more together… eventually outnumbering the Duggars, believe it or not).
I wouldn’t lump Diff’rent Strokes in with the rest of those, as Mr D already was raising and parenting Kimberly.
As for movies, I don’t know the name, so maybe it’s been mentioned already, but a while back I accidentally caught part of some piece of crap on TBS where Katherine Heigel and Josh Doumel (I’m sure I spelled at least one of those last names wrong, I don’t care) as single people forced to come together to raise a baby when the parents (each of their respective best friends) get killed.
I call BZZZZZZT on Kipling’s Captains Courageous. The captain of a fishing boat picks up a 15-year-old spoiled rich kid who’s fallen off an ocean liner, and considering his grandiose boasting and offers of lavish reward for a quick return on shore to be just so much deluded raving, essentially shanghaies the teen for a few months of hard labor at the Grand Banks cod fishery, which naturally Makes A Man Out Of Him.
The boy (Harvey) is a typical “green hand” aboard the boat, but his role is that of apprentice, not foster child. It’s well understood throughout by everybody that Harvey has parents of his own (although the captain and crew don’t have any conception of how wealthy and important they are till the end of the book), and that he will return to them when the season’s fishing is done.
Much closer to what the OP’s looking for are the Anne of Green Gables saga (though the orphan child there starts out as technically somewhere between a ward and hired help rather than a foster child), Rebecca of Sunnybrook Farm, and Understood Betsy.
“Konrad” an episode of WonderWorks on PBS. Lady receives a delivery that is an “instant kid” in a can.
My Two Dads, an 80’s sitcom about a girl whose mom dies not knowing which of two men she slept with was her bio-dad. (Of course, they have totally opposite personalities.)
In Anne of Green Gables and Rebecca of Sunnybrook Farm, the adoption of a child was planned out well in advance in both cases, so they’re not really cases of becoming parents unexpectedly, although in both the child who arrived wasn’t the one they’d hoped for. (In the first, the Cuthberts wanted a boy to help Matthew with the farm chores, and in the second, the aunts wanted Rebecca’s better-behaved older sister Hannah.)
There’s Carbon Copy starring a young Denzel Washington in maybe his first movie. (IMDB shows a couple of TV appearances and a “video” listed previous to this.)
Or The Apple Dumpling Gang if you like a Disney twist on the genre