Baby's mind and ICG's mind are fused.

There are TWO versions of this theory. One about SL1 & SL2 and another about just SL2.

Teaser.

In the new teaser, there are a few things that point to one thing. The sister. So, Baby is the main focus, but the sister is in the background.

• The file name is "iamstillhere". This matches up with the sister, because in the Private Room, in Ennard, the sister says "Everything is okay. I'm still here." This just ties to the new teaser, because not only is Baby still here, but so is the sister, in some way.

• The only color you notice in the teaser is green. The eyes are green. In Sister Location, the minigame shows Baby's eyes turn from blue to green. Since ICG has green eyes, then this is a connection. Even though the green eyes in te teaser don't prove the sister's importance, the fact that she was already given importance in SL, and that the eyes are the only thing you see here is important.

• In the teaser, Baby has pinprick pupils. And we've seen that before.

Glowing pupils

Glowing pupils.

The problem is, her pupils aren't white. How do we know this? Because the only green from the teaser comes from the pupils. If it was from the iris, not only would it be clearly glowing, but the eyeball would be glowing, too. That means you'd be able to see a white eyeball.

So this might mean that this is different. The white pupils were in possessed animatronic. However Baby may not be fully possessed, but ICG may somehow be apart of her mind.

It's possible that after Baby was ejected, so was ICG, who was apart of her.

So what does this mean for Sister Location?

#Sister Location

SL seems doesn't actually make a clear case for ICG's actual role in the game. There are might be implications that ICG is bound to Baby, but there's really not that much going for it. There is the part where she says "I still hear her sometimes." and it's the closest we get getting an actual idea.

Because if ICG is in control, or full control of Baby, then there are things to think about.

• Why she keeps calling ICG "a little girl", "her", etc. Pronouns that imply that she's not that girl.
• Why Baby talks as if she is a robot with a robot personality. "I was on stage once." and "People enjoyed my songs." and "I don't understand why PEOPLE do the things that they do."
• Why Baby says "I don't recognize you." if Mike is ICG's brother. Even though on Night 5, ICG talks to Mike as if she recognizes him.

So the two cases could be this;

• In SL, Baby's mind isn't merged with ICG, but it does once she becomes Ennard, or once she's ejected out of Ennard, or in between.
• In SL, Baby's mind and ICG's mind are one and the same.

EDIT: Credit to u/kenan89 for the the fused mind theory