Contextual Filter Path Alias
You can search for examples for each of your questions with &quot;backwards views&quot; and &quot;sibling views&quot;. In short:
Show children on parent page
Create a Views that lists Children, then add the Entity reference field as the Contextual filter (not Content ID). Configure it to Provide default value Content ID from URL.
No Relationships needed in this solution. Place the Block on Parent pages and don't forget to configure the Block contextual option, it won't work without it.
Show siblings on child page
Create a Views that lists Children. This time you'll need 2 Relationships and 1 Contextual filter, all sequentially linked.
The first Relationship is for your ER field (the Field on Child that references a Parent), this one introduces the Parent info.
The second Relationship is the &quot;backwards&quot; Relationship for the same field, it is named a bit differently that the &quot;forwards&quot; one. Make the second Relationship use the first Relationship. This will trace path &quot;backwards&quot; from Parent to all their Children.
Add the Content ID Contextual filter and make it use the second Relationship. Configure it to Provide default value Content ID from URL. This will tell Views which Child's page you're currently viewing and will match to the Parent and then Parent's children.
Exclude current Child from Views
Your current Child will also be listed in Views because it satisfies the same condition! (It's also Parent's Child, just like other Siblings.)
You can exclude it by adding the second Contextual Content ID Filter that doesn't use any Relationships, Provides default value Content ID from URL, but under the last section More also check Exclude.