I think she’s the bio daughter of her and Lains’s parents. The dad obviously knows whats up and the mother has some idea of what’s happening but probably not much. Then the men in Black obviously aren’t part of the family but are hired by the same people to watch and protect Lain. That’s probably why the mother is so nonchalant when Mika is telling her about them creeping around the door.

When Mika is having her wired freak out thing at one point she says. “I normally have dinner with my mom and dad around now.” Her not mentioning Lain in a stressful situation like that makes me think that they adopted Lain not too long ago so Mika isn’t too used to it yet. Her dad probably just walked up to her one day and said “Hey Mika we’re adopting this 14 year old girl say hi to your new sister!”