I have more than one example of this, one with a family with two children and one with a family with three children. In the relationships tab, I add the father and mother for multiple child character articles. Each child character article correctly shows their parents in the sidebar, however whilst there is an inverse "Children" header in the sidebar for both of the parents, only one of the children seems to be picked up and shown for each parent. I would have expected a list of all their children to be shown for both parents.
Steps to Reproduce
Create four character articles (as a minimum) for a father, mother and two children. Add the same father and mother in the relationships tab as the true biological parents for two children. Observe that whilst each child is displayed with their correct parentage reflected back in the sidebar, that at parent level, both parents only show as having one child under the Children header in the sidebar.
Ah! That actually looks like the solution. I just checked my parent articles and they are now all working, but when I raised this issue its quite likely that all of the children had not been made public yet. Seems fine now so happy for this to be closed.