@FereldanCherry
Sorry for late reply, fit of migraine :(
Nope, no step-parent/step-child relationship is created if you use the option that came with parenthood. The adult appears as "Betreuer" (engl. guardian) in the dependant's relationship panel and the dependant as "Angehöriger" (engl. relative, but again, kinda misleading) in the adult's panel. All romance options between the foster child and the guardian's biological children are available.
Just don't marry the foster mom to Vlad, because then she'd become the teen girl's step-mother for real.
I do that all the time in my pseudo-feudal rotational save: The nobility (and lately the rich commoners, too) fosters gifted teens with the express intent to marry them to their own offspring. I also set the summer camp instructors as foster parents for the duration of my class trips so that they can use the parenthood interactions on the students. It is such a handy feature :)
