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.