hi Purplebear

i havent read all of what everyone has said, so apologies if im repeating what others have said, Im a registered early childhood teacher, have worked in special needs schools in classes of children who were all autistic, and i have 3 children and 1 adult in my family/DHs family with mental disabilities (2 of them on the spectrum) so have a bit of experience in this area.. but Autistic spectrum is SO vast, from what you tell us without us knowing the child, she could just be a very lively, quirky child, who will grow out of her funny ways- or use them to become a genius in the future-who knows? or she could be somewhere on the spectrum.. its a very complicated thing to diagnose. how close are you and your sister?i guess all relationships are different, i know for me and my sister we have a close enough relationship that i could say something to her about her children if i was concerned and vice versa..but i know all families arent like that.
I did have a talk to the Dr about it but she said it was common for parents to have suspicions about their kids at age 3ish, and that it usually died out slowly as they came to school age. But if anything is going past school age, then it's a problem.
Now I am not at all stymieing the possibility that there could be something genuinely wrong with this little girl. But I do think that you need to chill about this. If there is a real problem then she will get picked up and placed in the system. In the mean time, I agree with the quote katnik posted. Parents don't want to hear that there is something wrong with their kid from someone on the outside. The best you can do, is be supportive and provide help and recommendations.
see, im sorry but i have to disagree with the above..getting to school age without a problem being picked up on is not a good thing..if the child gets to school and no support has been sought after, this is when a child often VERY quickly gets labelled with things like "trouble child" and slips under the radar. not saying always, but from experience, ive seen it happen way too often..including my DHs little sister who is now 12, she was labelled a disruptive kid who couldnt pay attention in class until she was 8, when she was finally diagnosed with asbergers, and now is a totally different child, thriving and learning with her teacher aide, fully funded (another thing with children who need support but arent diagnosed as anything is that the school and parents get no funding for anything the child needs...my MIL paid out of her own pocket for years for a teacher aide because little one couldnt cope and no one listened to her when she said she was more than a naughty child..)
and no, someone wont necessarily pick up on it and pass them on...often everyone leaves it to everyone else to do..kindys etc dont always get it right, so you cant rely on them always..so the best people, if there is real genuine concern, are family who can support each other through then entire ordeal..because it IS quite an ordeal when you are talking about diagnosing mental disabilities.
so at the end of the day, its up to you..if its keeping you up at night, and your relationship with your sister is tight then maybe you should say something?if not, maybe wait until she is 4 and then you could say something about being ready for school and coping etc, and bring it up that way?
GL hun, you are a good sister for seeking advice like this
