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To be honest, I vaguely remember this one scene from the movie (I was like 5 when I saw it I think).
The scene depicts a teenager spying on this stepmom/dad's girlfriend along with a girl (possibly girlfriend) because he has a hunch something weird is up with her and he sees her in the backyard transformed into her alien form.
The movie probably belongs to the horror-scifi genre since the alien form was scary (at least to the 5 year old me).
The movie is possibly from the late 80s/early 90s.
Stepmonster (1993).
From IMDb:
Todd Dougherty has an imagination driven by comic book. So when he accuses his soon-to-be stepmother of being a monster, no one believes him. When neighbors start turning up dead, it becomes a race against time for Todd to stop this hideous, carnivorous creature from marrying his dad and ruining his.
The stepmom can't tolerate the violin songs life.
Depending on how well you remember it, this might be My Stepmother is an Alien from 1988. The main teenage character in that movie is a girl (Alyson Hannigan), but she hangs out with a boy (Seth Green). Her father (Dan Aykryod) is a scientist, who ends up meeting an alien (Kim Basinger).
This review from SyFy wire hits most of the plot points, although in a very negative tone.
