Is it permissible to talk with children about an injustice their sister is experiencing from her husband in her and her husband's absence, and is that considered backbiting?

If you mentioned your son-in-law with something he dislikes in his absence, you have backbitten him with unlawful backbiting, for the Prophet, peace and blessings be upon him, said: "Your mention of your brother with what he dislikes." His injustice to your daughter does not justify this backbiting. The proper course of action is to advise him gently and wisely, and remind him of Allah.