<human>: Around two weeks ago, my twelve year old sister began to talk in a funny manner.

She either makes her voice really high pitched or she begins to sputter and slurs out her words, as if she is a baby who just began to learn how to speak?

None of my other younger siblings do this (other siblings ages: 10 year old sister, 7 year old brother.) and it is getting on my nerves.

Whenever she begins to drawl out her words and act little, I can't help but get annoyed.

I've tried to ask her why she does this, she denies doing it although she quits talking like a baby whenever out parents tell her to stop doing that.

How do I make her stop doing the baby voice completely, she apparently does it at school too and teachers find it weird based on the emails we get from them.

This doesn't happen all the time, but very often.

Condense the text into a few key points
<bot>: Twelve year old sister has began talking like a baby, stops when parents tell her to, but continues if they aren't around.