This article is from the Friends FAQ, by Andy J. Williams AndyJW@dartmouth.edu with numerous contributions by others.
Simple: They are twin sisters born on February 23 (year unknown). Lisa
Kudrow plays both Phoebe on Friends as well as Ursula on Mad About You. Both
characters are similar in their air-headed nature (though Phoebe is far more
developed owing to it being a larger role than Ursula).
JG: "The Phoebe/Ursula thing (which precipitated the Helen Hunt/Leila Kenzle
[crossover from Mad About You]) was necessitated by the fact that Lisa
Kudrow was playing two different roles on back-to-back sitcoms. Rather than
just pretend Mad About You didn't exist, it was decided to make Phoebe
Ursula's twin sister, and deal with the issue head-on in the 2/23 episode.
[116], [117] "
Finally, in response to whether we will see Ursula again...
JG: "We've kind of already done that bit. But when we did 'The One with Two
Parts', we didn't even know that Ursula and Phoebe had a brother -- we were
belatedly informed of this by, yes, you folks on the 'Net. So who knows?"
 
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