Personally I assume that some would think that a family relationship can be both fixed and fluid because you are born into a family (making it a fixed relationship) and cannot change that fact or the family you’re joining... that said as you grow you are able to able to make the decision as to whether those family relationships remain in tact or not.... just as you are able to reinitiate a previously “off” family relationship after whatever trauma and healing was necessary... thus making the family relationship fluid.