Is informing one's sister that her brother's wife is ill-mannered, instigative, and lazy considered backbiting and a major sin?

Your mentioning the faults of your brother's wife and inciting your sister against her combines both backbiting (gheebah) and tale-bearing (nameemah), which are major sins. A tale-bearer (nammam) is someone who transmits statements between people with the intention of causing corruption. The Prophet, peace and blessings be upon him, said: "The worst among you are those who go about with tale-bearing, causing corruption between those who love each other," and "A tale-bearer will not enter Paradise." Tale-bearing differs from backbiting in that tale-bearing requires the intention of causing corruption and conveying a person's situation to others, whereas backbiting is mentioning a person in their absence in a way they would dislike.