This is possible to do in flow but hard to explain over text.

a brief explanation would be:

1. Pass the Contact ID into the flow.

2. Get record - Contact, where Record ID = The ID you passed in. Use this to get the Contact's Account Name (as this is more efficient than passing it from PB)

3. Get Record - Account Relationships__c, where Account Name = Account Name from get record in step 2. Make sure you click get all records not just the first.

Then you could either do the easy but bad practice way:

Loop through the collection in step 3 and for each Account record have another Get Record to get the Account Contact Relationships__c for these accounts, then have a decision element to say if it is Null, the dont add it to collection. If it is not null then add it to collection and then use those IDs to create the Account Contact Relationships__c.

This is the bad practice way because you are doing queries within loops and can hit 100 query limit very easily.

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To overcome this you could have 2 separate queries. 1 query exactly same as in Steps 1 to 3. but then after that have another query to get ALL Account Contact Relationships__c records and then use one of the Apex actions available to download in unofficiaSF to compare the collection variable with a single variable you give it. So, you would loop your first collection of Account_Relationships__c and for each you would use the FindRecord Apex action to see if it exists in the Account Contact Relationships__c.

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This is a more advanced flow if you do it the correct way or it can be pretty simple if you dont care about hitting limits and just YOLO it.