How to tell my son she is moving in?

You need to unpack those cartons before your son comes home from school today. Having her belongings sit visibly in the hall while he still calls her aunty means the adults have moved at a pace that left him entirely behind, and walking into that physical reality without warning will likely trigger immediate resistance from him. Sit him down before dinner and speak plainly about the change. You can say that she is going to live in this house with the two of you from now on, and that things will be different, but you are still his parent and that will never change. This conversation will be uncomfortable and he may cry or shut down, and you will have to sit through his upset without rushing to fix it or getting defensive. Give him a small space in the home that remains entirely untouched by her arrival so he retains some sense of control over his immediate environment. Transitioning into a blended household with family involvement brings its own pressures, but letting a child discover a new co-resident through stacked cardboard boxes in the corridor sets the relationship up for avoidable resentment. Slow down the domestic merger slightly to give him time to adjust his vocabulary and his expectations.