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Lain's "mom" is the most distant from her, almost never speaking and treating Lain with indifference. She apparently likes Lain's "father" Yasou (or at least likes making out with him ^-^) and is very critical towards Mika. (The webmasters at The Cyberia Cafe http://www.cyberia-anime.com/
also joke that the only other thing she likes besides Yasou is rice.)
Miho is, of course, not Lain's real mother and she may not even be married to Yasou. The whole of the Iwakura family are just actors that will go their seperate ways when Lain figures out who Lain is.