Answered: Impostor poses as a heiress

American, 90s, maybe made for TV

In a small town, there's a big mansion with a rich family. A girl in her 20s knocks the door, and introduces herself: she's the long-lost relative that disappear time ago. Everybody is skeptic, but she knows the answers and she's very charming. Besides, the family is falling apart, and nobody is in charge. There are two children: a girl around 6 that has some illness that keeps her constantly in bed, and a nerdy boy around 10. He wears glasses, is lonely, irritable, and never uses money, 'cause the every time he buys something the shop sends the bill to the house. The impostor befriends the guy, pities him and teaches him a bit to stand in the world, but he's the one that never believes her identity, mostly out of stubbornness. The the girl leaves the house and leaves a note to the boy, explaining that she was lying, that she knew the missing girl story because she worked as a nurse for an old lady that coached her on all the details, and that now it's up to him to decide is snitching on her or not.