I feel for your sister. She had a very stable lifestyle going where she was the center of attention - and it was completely yanked away from her --instantly! That's tough. At that age it very hard to cope with.
When is your sisters birthday? It maybe that she is simply too young for an all day class. If she was born after Sept., she could be one of the youngest in the class.
If she is age appropriate, the behavior is not all that unusual. I have seen kids cry for about two weeks (at the worst), and then bam! everything was fine. They do adjust.
It is really important for her to find a friend. It can make a huge difference. If you are still around, look for anyone she sits with at lunch or plays with at recess. Then try and get them together after school or on the weekend. One friend will make an immediate difference. The gummies at the end of the day are no big thing. Kids of that age deal with immediate response. The teacher would do a lot more good by handing them out during the day. In fact her method is kind of destructive to kid's egos. What would make a difference is if the teacher would greet your sister by name and with a big smile when she came to class. If the teacher has been teaching kindergarten for (say) more then 5 years, she should be able to give you some very good advice on how to handle the situation.
By the way, is your sister used to using crying to get her way at home?
Also, you said, "and to this day i cant leave her at school or she will cry all day." If that means that as soon as she starts crying, you take her home - that is absolutely the worst thing that you can do. It tells her that all she has to do is to cry and she will go back to being the center of attention at home.
Speaking of being still around. I would not appear in the classroom any more. You know whats going on. You being there will not help the translation to school any longer. And in fact will probably hinder it.
I don't know how old you are, but I congratulate you for taking the time to be with her and to make this post. You are a great sister. I would not take her to a doctor quite yet. Try the things I mentioned - give her some space. Best wishes!
My mom started a full time job about a month before she went to school! thats why i sat in for her and i didn exactly what u said and it worked for about 3 days and it went back to she same! except it feels like its getting worst not to mention the teacher gives the class gummie treats at the end of the day for good behavior and starting just las week the teacher is starting to take some gummies away from her for crying! i feel so bad for her when this happens! theres kids that hit other kids all day and them to get a gummy takin away but i dnt think her crying amounts to that kind of treatment! crying is far less then hitting another kid 5 times a day!
Does your mom work? If not, can she go to school with your sister and sit in, for a few classes? Your sister might get a lot of comfort from her mom in the room. She could just be around, quietly in the back, and your sister would be able to look back and see her and could tell from the look on her face that everything was perfectly all right.
Your mom should be talking to the teacher about this situation, and then the principal. If nothing else, she could talk to a child therapist. It might be as simple a thing as changing rooms and teachers, or it might be more complex.