There isn't one.
You're thinking of 'April in Quahog'. FOX released promo pics featuring The Smiths and The Griffins but the crossover turned out to be an April Fool's joke.
Although Francine did recently appear in a mock American Dad intro w/ Joe Swanson in the place of Stan, in the episode 'Foreign Affairs'
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Wrong show, Francine joins the Ladybugs on "American Dad"
Family Guy Petoria Episode.
The 255th episode aired on Family Guy was "Peter's Sister"
No.
It is an episode of Family Guy