Yes there was an episode where they showed Niles wife. You could only see her from the rear. You never saw her face. I do not remember what season this was or what episode it was either.
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= The episode's called "Give Him The Chair!" =
The name of Niles first wife is Maris.
Niles was first married to Maris, then later towards the end of the series he married Daphne Moon.
Season 4, Episode 18.
Maris Crane, Niles Crane's wife, fled to an island owned by her family to avoid being prosecuted for the the murder of her Argentinian ex-boyfried.