What is incumbent upon me if I suspect that I placed the zakat of one of my sisters in the donation box instead of the zakat box, and the amount of my own charity in the zakat box instead of the donation box?

Scholars have differed on the recipients of Zakat al-Fitr, holding three main views:

The majority of scholars: It is permissible to distribute it among the eight categories mentioned in Surah At-Tawbah (9:60). The Malikis and some scholars: It is specifically allocated for the poor and the needy only. The Shafi'is: It is to be distributed among the eight categories or those among them who are present.

If the donation box expends funds for purposes related to Zakat categories, then there is no blame upon you. However, if it is for purposes other than Zakat categories, then you have erred, and you are liable to guarantee what you deposited, because a trust becomes guaranteed when it is neglected or intermingled. If you do not know the amount that was placed in the wrong place, then you must endeavor to determine it.