Is a sister's sharing of her marital problems with her sister for the purpose of consultation and seeking advice considered forbidden backbiting (gheebah)?

If the sister's intention in speaking is merely to vent, then this is likely not free from prohibited backbiting, which is mentioning what the husband and his family dislike, as the Messenger of Allah, peace and blessings be upon him, said: "Your mentioning your brother with what he dislikes." However, if the intention is to seek advice and consultation, then backbiting is permissible to the extent of the need, and it is not permissible to elaborate, because the default ruling is the prohibition of backbiting, and certain cases have been exempted for a legitimate benefit, so the permissibility revolves around the hoped-for benefit. If the sister continues to mention what is unnecessary, or if you have no opinion on it, then ask her to stop, and explain to her the danger of backbiting.