I suspect that you expect an answer like "children of a country-music duo" - right?
Tammy should be about 30+ by now, and
Crystal (where I live anyway) could be her aunt, so 50 at the very youngest. The only
Wanda I ever knew had a Polish father and a Scottish mother; her brother's name was
Roderick. So their names weren't chosen according to fashion but to match the opposite-sex parent's origins.
I dislike
Crystal for its costume-jewellery vibe; the only
Tammy I've ever known is delightful but I had the misfortune once to see a
Debby Reynolds movie about a ditzy blonde
Tammy which put me right off the name for life, though I could just about face
Tamsin. And I've got nothing to connect
Wanda with, except that one little girl long ago. I suppose I'd think that each sister had probably been named after someone really special. Or, that it's a free country!