Try this. Go buy a pack of appropriately sized pull-ups or Depends and a couple of tween sized onzies/footy PJs. Next time the 12yo goes into toddler mode smack your hand down loudly on the table, tell her to knock off the baby shit then toss the packet of diapers at her and tell her to go put one on. Since she is behaving as a baby she will be treated as one then march her to her room and tell her not to come out unless she is diapered up.

Then turn to your bride and tell her that you will no longer tolerate the 12yo behaving as anything other than a 12yo and every bit of her clothing except for diapers and tween sized onezies will be thrown away and she can deal with her school peers dressed according to how she behaves.

That ought to just about take care of it.

My SD10 used to do this. She talks completely normal to us, but then when she used to talk to her BM on the phone, she would go into whiny baby mode. DH and I would not allow her to act like a baby when she was here. She would get in trouble. She also used to crawl and hide under furniture or cry if you didn't pay attention to her. MIL and BM both insisted it was because she had development issues and needed coddling. DH and I refused to allow the behavior, and you know what, it's been 2 years since she has done any of those things around us. I think it is an attention thing. It won't stop until her mom stops encouraging it.

Just a side note, our SD10 has no contact with BM, but she did up until a few months ago. She would always revert to this behavior on the phone. It was her way of getting pity or being spoiled by her mom.

Well, it could have to do with separation anxiety. I'm going through this with a teen that's 15, the whole clingy toddler voice stuff. She'll get on her mom's lap and ask to stay home from school or whatever. Drives me insane, because she's having a hard time being her own person without her mom doing everything with her. Luckily, my wife is starting to realize this.

I dunno how to really bring it up without looking like an a-hole, so I basically quit pointing it out and now it has gotten so much worse that my wife has to do something about it instead of telling me I'm just being mean. Disengaging really works in my case, because it shifted the blame from me to whatever it is that's going on.

And, my kid's therapist just talks to the kid about random crap all the time and has since they met a few years ago. Does nothing to address the issues, even if my wife talks to her about stuff. Tells the kid it's okay to stay home and do nothing all day since she had a hard life. It's so ridiculous, people treat people with kids gloves too much and that's why they have a hard time acclimating to the actual world.

This happens with my 11yo SS.. And thumb sucking, he reverts into this annoying lispy toddler voice while keeping his thumb shoved in his gob.
It's mainly his Mother he speaks to this way though because I don't tolerate it.

OH GOOD LORD that would drive me insane. My 13 year old SD is super immature for her age and she is coddled by her BM to the point of exhaustion. I think her BM is refusing to let her grow up and her anxiety is rubbing off on SD. It's incredible frustrating.

This crap should be intolerable and have significant and completely unpleasant consequences. Each and every time it happens.