Is informing the husband about what is said concerning his wife in his absence, or about the injustice inflicted upon her by his sisters in the home, considered backbiting (namima)?

Backbiting is defined as conveying the words of others with the intent to cause mischief, and it is religiously forbidden and blameworthy. What the questioner mentioned does not fall under the definition of backbiting. It is important to note the prohibition of spying, and that good companionship includes being kind to the husband's family, overlooking their mistakes, helping them maintain kinship ties, and gently enjoining good and forbidding evil upon them.