Really good question, I talk to my students all the time about this in my tutorials.
You call it a family of sounds, here you have a mother and a father, via "epiano" and "bass", to find the other complementary sounds that will go with it well, it should have somewhat similar timbre, this does not work all the time but for the most part you're looking for compliments, not so much as contrast, because a contrasting sound sometimes distorts the intended emotion of the track, for example when folks are listening to a soft and pleasant beautiful symphony, it would be awkward to include a gritty heavy saw sounding synth bass. It would be more applicable to add a Tuba, basson or something in this range, which has the same type of timbre as the gritty bass synth, but more subtle sounding because it compliments the family of orchestra sounds.
You want a heavenly pleasantly musical piece, try a japan flute, chimes, woodwind sounds, very soft strings, clarinet, oboe etc.
Looking to evoke some dance emotions, use nice gritty synth sounds, thick pumping bass, 808 kicks, snappy snares, orch hits, fat synths chords and such. These sounds strikes the nerves in a motivating way. To fully understand you have to see this in action, I'll do a tutorial tonight addressing this subject on my site.
