Sorry, I noticed that this is your sister, not your child, Agies mom.
Is they a child younger than her in the home, a baby, toddler? If so, she may be using baby talk to get attention. Its very likely that this is attention-seeking behavior. Its very good that she is not anxious or insecure because you would have to deal with these issues?
Don't make too big a deal of the babytalk, though it may be annoying to you. Ignore it when you can. Again if she tries to communicate directly with you in a baby voice, gently tell her you want to her her big girl voice. Find big girl activites you or your partner can do with her without the presence of any siblings to make her feel special. A girls shopping trip, a trip alone with her to the park etc. Eventually the desire to be a big girl will win out of the attractivenss of being a baby! Best wishes...
I noticed her starting to talk this way when she started preschool, at 4 almost 5.
We haven't had any trauma in the family that would set it off, we did move to a newer house but there wasn't any major change to effect her, she went to the same school, same friends, etc.
I wish i could say she appears anxious and insecure but she can be very outgoing and not the least bit shy, when she was younger she wouldn't talk this way, which is why it's so strange that she's talking this way now.
I'll try your suggestions and hope she does grow out of it sooner then later.
I doubt this 6 year old girl has a good grasp as to why she is displaying regressive behavior. She is a young child in the scheme of things. When did she start doing this? Was there any signficant changes in her life, school or home environment? Its not unusual for kids to regress to an earlier stage of development when are subject to stress i.e. thumbsucking, toileting accidents, baby talk etc.
I agree with koukla, tell her you don't understand her when she baby talks and that she needs to talk like a six year old.
Additionally, does she appear to be unusually anxious, nervous or insecure? If she is having difficulties with anxiety or if she is insecure as compared to most kids her age it may be worth consulting her pediatrician or family doctor in order to gain some perspective. Likely this is something she will develop out of.
Best wishes...
You can ask her to talk to you in her big girl voice just like the teachers do. In addition, if she continues to talk in that way, say "I'm sorry, I can't understand you." She will stop using the voice with you - eventually. Then maybe you can tell mom and dad how interesting you find it that she no longer talks to you in the voice.
She will not inherit a lisp from talking that way, if she doesn't have one in the first place. I do believe she will outgrow the voice when she gets older.
Has she always done this or did she suddenly start to do it? If it's not a result of some sort of trauma (something upsetting happening--abuse, a loss of a family member, divorce, etc.), I would think it will stop on its own. From my experience, kids whine and talk baby talk because it's working for them. She gets some sort of satisfation from it (positive attention, a giggle, smile, etc.--she may even find it amusing to see you embarrassed by it). I imagine she'll stop when it stops working for her (when your parents no longer think it's endearing and stop responding positively, when kids tease her about it, etc.).