Had a chance to re-watch Glass House (2001) because I remembered enough of it to think it might be Girls Underground, and I was right! And while it doesn’t take place entirely within a house, as the title suggests it does revolve largely around a house, and I happen to really like that subset of GU stories.
Ruby, 16, and her little brother go to live with a rich couple in a large house made mostly of glass, after their parents are killed in a car crash. The couple quickly start acting creepy in different ways. Ruby seems to find help in her parents’ estate lawyer, but he eventually betrays her (though he was well-intentioned). Other than him, her only companion is her brother (who is also the family member she must rescue), as her former friends quickly dismiss her when they don’t hear from her for awhile. The husband especially becomes the main adversary, while his wife is the lesser problem (and eventually out of the picture entirely). He is being threatened by mobsters over a debt and seeks to control Ruby and her brother’s fortune. It also starts seeming like he may have killed their parents, and been plotting the whole thing from the beginning.
Eventually Ruby confronts the husband, and he throws them both in the basement (underground!). But they escape, and there is a final showdown, where Ruby prevails.