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Answer:
In the main idea or point used in an essay. It is usually made at the introductory paragraph where supporting paragraph are then used to support the statement. You know how we write topic sentence in a paragraph.. A thesis statement is almost the same but in an essay that statment if focused on throughout the whole essay.
Since its a sibling, and siblings tend to be in a person's close circle of people, informal speech is used.
I guess it would either be mood or syntax