She will adjust.
They all do. Usually.
Some adjust quick. Some adjust less quick.
In the end, they all adjust. Usually.
The Teacher... is used to this.
She only goes 3 days a week for only 3 hours.
I would NOT... have the older sister be her recess playmate.
Older sister, should be able to have recess too... with her friends.
And it just makes it harder, on the younger child. To have older sister go away at the end of recess.
How does the Teacher handle this????
Each Teacher & school... has different manners of helping kids adjust.
Ask the Teacher.
Get the book "The Kissing Hand." It is a real cute great book, about going to school. Just for this purpose.