Mika Iwakura 岩倉 美香 (Iwakura Mika)
Lain's older sister, sixteen, is the most normal person in the house: a little flashy, out late with friends and mildly fed up with her odd younger sister. She mocks Lain for talking to her NAVI, spies on their father setting it up and is the first to meet the Men in Black at the door, threatening to call the police. Ayako Kawasumi voices her. Mika exists only in the anime; the game's Lain has no sister. Layer 05, 'Distortion', is largely her episode, and Konaka told HK he wrote it in bad heat and now thinks it came out too theoretical, wishing he had written it more by feel.
More (spoilers)
In Layer 05 Mika is hounded by messages telling her to 'fulfill the prophecy', sees her own double at home and breaks down. For the rest of the series she sits slumped and makes noises, later sounding like a modem or fax. Ueda explained at Otakon 2000 that she became a victim of the Wired: her self got stuck in between, so her body behaves like a puppet. She vanishes with the family in Layer 10 and returns to normal after the reset.
Sources: en.wikipedia.org archived · ja.wikipedia.org archived · animenewsnetwork.com archived · konaka.com archived · lawmune.net · lain.wiki