Is the salary of a sister who works in a company with male and female employees, while adhering to Sharia guidelines, forbidden? And is accepting her gift forbidden?

There is no harm in your sister's work if it is permissible and she performs it as required, then she may benefit from what she earns from it. Unlawful mixing (ikhtilat) is the gathering of men and women with the presence of a religiously prohibited element, such as ostentatious display (tabarruj) and speaking in a soft, alluring manner. It is not merely the presence of men with women in one place that is prohibited, except with the commission of a religiously prohibited act. Scholars have differed on the issue of uncovering the face and hands, and the preponderant opinion with us is the obligation to cover them.