To answer this question, you need to try to understand the workings of mediumship.
Mediumship is a person acting as a go between. Between spirit entities and humans. The medium acts as the 3rd person, passing on the information from the spirit to the human.
When working as a medium you have to learn to lift your consciousness to a higher level. This is so that you can pick up the communication coming from spirit. It is not easy to do if you cannot reach higher. It is impossible to do if your mind is full.
Your mind is always working. It has to be unless you are dead. It is part of the human makeup. So there is no way you can empty your mind and stop it working. That is why you need to go to the higher level. Your mind is still there but what is coming in feels different.
If you know someone very well, as with a relative, you have in your mind all the things you know about them. It is impossible to cut them out. Therefore, they get in your way.
You need to form a channel for the spirit communication to come through. if it is already blocked with prior knowledge, then you cannot differentiate the info coming from spirit to that coming from your own mind.
The best mediumship comes about when neither sitter nor medium know each other.