Must the money spent on the deceased maternal aunt from her own wealth (cost of medications, treatment, maid's wages) be returned to the heirs, and is the questioner alone obligated to return the money given the presence of brothers and sisters?

You should not demand the return of what you spent on your aunt, with her permission, for her treatment and needs. However, what you spent during her lifetime without her permission or for non-essential needs, and what you spent after her death without the permission of the heirs (such as feeding relatives and holding dinners), you are liable for and must return to them if they do not forgive you. Your siblings do not share in your liability for this money, because it became obligatory upon you due to transgression, not kinship.