I would like to help. When you say interfere do you mean they are trying to help in their own way or are they trying to actively ruin relationships?
Yes, I am familiar with this and it is a terrible thing to be done. Mainly this is done between divorced couples. Usually one does this because they are insecure and feel threatened. They are afraid that the child will love the other person more. They also do it because they want to hurt the other person. It is a terrible thing to do and hurts everyone involved especially the child. You asked how can they be shown that lover doesn't subtract it adds. This depends on the specific type of situation and the ramifications around it. My advice may be different in regards ***** ***** based on their specific actions and which family member it is.