Being neutral means, that you keep an outside perspective on the client, on their issue and on what happened in their family. You do not sympathize with the client, you do not pity your client, you do not agree with opinions of the client about their family members.
It means that you can easily and consciously move into the field of the client's family when you need to perceive or sense what is there — and you can move out of that field instantly again after you have made your observation.
Being netral means: you do not step into the emotions of the client. A clear sign of losing neutrality is when you take over the emotions of the client. Or when you experience strong emotions that would be appropriate for the client to feel, however the client does not show any emotions.
You need to learn to recognize when you lose your neutrality; when you are pulled into the field of the client's family. Now this self-awareness is actually quite difficult. It is a discipline that you probably will continue to develop throughout your career. It is like recognizing that you are dreaming while you are dreaming.