Display a related list of a child object on another child object's record page

To make this modification, utilize the 'Related List - Single' component.
 

1. Navigate to Setup | User Interface | Lightning App Builder
2. Click Edit on the Opportunity Lightning page or create one.
3. Add the Lightning component called Related List-Single.
4. Customize the component with the following values:

5. Save and activate.
   1. Verification of Organization-Wide Defaults (OWD) and Field Level Security on the Parent Record is required.
   2. Availability of the child Related List on the Parent Record is essential.
   3. Inclusion of the lookup field on the page layout of the child record is necessary.

The result, you should have a component which is named "Contacts for Parent Account" displayed on your Opportunity detail record page which will display all Contact records associated to the Account and this should apply to all Opportunity records. 

More Info:

Standard Lightning Components
​​​​​​​Salesforce Support YouTube Video ​​​​​​​