The Family Access Key provides any Parent user provided with that key access to all siblings within that family unit.  This can be linked to both Mother and Father users who have been provided the access key under different logins, if parents of the same family choose to have separate user accounts for the Portal - as you see in the scenario provided. 

If you have a family with split custody, requiring that Access to students be given on a parent-by-parent basis, then you want to either: 

a) for split custody where ALL siblings are to be accessible to both parents, however, linked to different access, generate Separate Family Access Keys.  In this scenario, you then provide each separate Key to each parent; OR

b) for split custody where different siblings have different custody arrangements, you will want to generate Student Access Keys per student for each user to be linked.  You then provide the per-Student Key to each parent as applicable.