Is it permissible to complain about a grandmother's mistreatment and harm among family members without her knowledge, and would this complaint be considered backbiting?

The basic principle is that mentioning a person's flaws in their absence constitutes forbidden backbiting (gheeba), unless there is a legitimate Sharia-compliant purpose, such as seeking justice (tazallum), asking for a religious ruling (istifta), seeking advice (istishara), or warning others. Therefore, mentioning the grandmother's wrongdoings in her absence is forbidden except for these reasons. It is advisable to be patient with her misdeeds, counter them with kindness, and overlook her slips, especially given her advanced age. Indeed, the wise person overlooks slips, and exaggerating them is not a characteristic of noble individuals.