These are parentheses: ( ) Don’t use them when brackets are called for.
These are brackets: [ ] Use them to enclose information in a quotation that is not part of the original. For instance, “A Ken Burns documentary shows him [Mark Twain] to have had several financial setbacks, primarily of his own doing.”
Brackets are also used following an error made by another, showing that you didn’t make the mistake yourself: “Trump once said that an accusation against him was unpresidented [sic].”
Sic is Latin for thus or so. Put it immediately after the offending word.
[Sic] is Latin and means so or thus. It is used, always in brackets (not parentheses), immediately after an error in either speech or writing that is being quoted. It indicates that the writer who has used that quotation knows that the word immediately before [sic] is an error. It is used so that you, the reader, will know the error was not made by the writer you are reading but by the writer being quoted.
Incidentally, [sic] never indicates that any material has been omitted; to show that, you use ellipses (… at the beginning or in the middle of a sentence or …. at the end).