are you bringing every character to 20 - also the Human siblings?

Absolutely not. I've committed to training the vanguard's twin just last weekend, but no more.
There simply isn't enough variety in the ways to play them to warrant it. Vanguard is the only one I would respec weekly, switching from combo build to SP Nova build every time. For all the incidental incidents I want to play another hooman differently I'll use respecs.

I frequently get Character Respec card from Advanced Packs, and currently have 22 of them.

And since weapon grindfest has become much more severe than ever, I don't have the luxury to level up the other copy of Human classes. I just use Char Respec whenever I feel like it.

I only have the ladies leveled to 20.

Why would I play with some dude if I can do it with a cute ponytail?

I am talking about levels - not ranks.

it takes ACTIVE effort to bring a secondary to level 20.  That's why I asked.

↑↑↑This, game f***ing loves giving me respec cards. I have over 30 now(stopped keeping track of the exact number but I'm pretty sure its close to 40) and whats funny is that I have 2 characters lvl 20, 3 close to 20 and that's it. If I want to test something out I'll just use one of the many respecs I get out of seemingly every other pack I open.

Been done for awhile. I really enjoyed leveling everything once we got experience boosters. Bought allot of gold experience boosters to supplement what I got in packs. Now the situation has changed with MF being useful w/ mods, characters, and weapons available in the item store. Not sure if I'd still be buying boosters today.

Probably, I still play the bronze and silver Apex missions.

He's a fun kit to play, you're missing out.

Hold the line JRandall0308 HOLD THE LINE!

I level the second human when the game asks me to do Bronze or Silver Waves.
I've put some levels on H!Sol, T!Sol and B!Scrapper playing Bronze during the Combat Mastery challenges, and might end up with them going all the way to twenty that way.
I start levelling every other kit once it gets close to X in the rank on Silver to see if I can play Gold on it, and i am going to leave the rest of the humans till I run out of kits with enough ranks. 
The only chars I don't plan on levelling are all UR and all infiltrators. UR because I will go nuts with always staying with partial points, and infiltrators because I don't understand how to play them and am terrible at playing them.

They're really quite easy. Shoot till you get shot, then cloak till they turn around. Rinse and repeat.

I am a terrible shot, so I prefer power classes. Also, as much as absolutely love doing objectives, I am terrible at platforming, and being an infiltrator makes me feel that everyone just assumes I'd do it, and it's very embarrassing running around in circles cloaked trying to find the dumb thing or place we're you can hop up. The only thing I love is being able to revive. Anyway, unless they get a kit that is simlar to the old Asari infiltrator with a DoT dark channel or whatever it was called and warp to xplode it in addition to cloak, I'll just end up continuously embarassed when trying to play one.