Hi all,
I want to show in child accounts, the related contacts of the parents accounts too. I almost solved this with a Visualforce Page and a Controller, but the related contacts are shown only on the parent account and the current account contacts. 
How do I show the related contacts of the other parent child accounts contacts on the child account too?

Example: I have parent account A, and child Account B and C. When I click on the Account C, i want the visual force page to show me the related contacts of the Account A + the related contacts of account B. 

Thanks in advance for the help.

Here is my Controller:

public with sharing class AccountHierarchyController {
    public Account acc { get; private set; }
    public List<Account> accountList { get; private set; }
    public ApexPages.StandardController controller { get; private set; }
    
    public AccountHierarchyController(ApexPages.StandardController controller) {
        this.acc = (Account)controller.getRecord();
        this.accountList = new List<Account>();
        
        Account baseAccount = [SELECT Id, 
                               ParentId, 
                               Name,
                               Parent.Name,
                               (SELECT Id, Name, Phone, Email, Title from Contacts),
                               (SELECT Id, Name FROM ChildAccounts)
                               FROM Account 
                               WHERE Id = :acc.id];
        accountList.add(baseAccount);
        
        Account currentAccount = baseAccount;
 
        // Traverse the hierarchy upwards.
        while (currentAccount.ParentId != null) {
            currentAccount = [Select id, Name, ParentId, Parent.Name, (Select Id, Name, Phone, Email, Title from Contacts) from Account where Id = :currentAccount.ParentId];
            accountList.add(currentAccount);
        }
        
        // Traverse the hierarchy downwards
        Set<Id> accountsToQuery = new Map<Id, Account>(baseAccount.ChildAccounts).keySet();
        while (accountsToQuery.size() > 0) {
            List<Account> thisLevelAccounts = [SELECT Id, Name, Parent.Name,
                                               (Select Id, Name, Phone, Email, Title  from Contacts),
                                               (SELECT Id, Name FROM ChildAccounts)
                                               FROM Account
                                               WHERE Id IN :accountsToQuery];
            accountsToQuery = new Set<Id>();
            
            for (Account a : thisLevelAccounts) {
                // Add this Account (with its Contacts) to the list.
                accountList.add(a);
                
                // Add this Account's children to the query for the next level.
                for (Account child : a.ChildAccounts) {
                    accountsToQuery.add(child.Id);
                }
            }
        }
    }
}

Here is the Visual force page:

<apex:page title="Contact" standardController="Account" extensions="AccountHierarchyController" lightningStylesheets="true">
    <apex:outputPanel id="cont">
        <apex:pageBlock title="Contacts">
            <apex:repeat value="{! accountList }" var="a">
                <apex:pageBlockSection title="{! a.Name + IF(NOT(ISBLANK(a.ParentId)), ' (child of ' + a.Parent.Name + ')', '') }">
                    <apex:pageBlockTable value="{! a.Contacts }" var="con" id="conlist" title="Contact">
                        <apex:column value="{!con.Name}"/>
                        <apex:column value="{!con.Phone}" />
                        <apex:column value="{!con.Email}" />
                        <apex:column value="{!con.Title}" />
                    </apex:pageBlockTable>
                </apex:pageBlockSection>
            </apex:repeat>
        </apex:pageBlock>
    </apex:outputPanel>
</apex:page>