Okay you want to keep talking about this. I have said on this web site, even recently, if the plain sense does not make sense, you should look for a figure of speech. A metaphor, and Idiom, a simile etc. A custom or a known tradition may play a role in the interpretation and can sheds light on the passage but must be validated. (More on that in a moment.)
Context, many times will determine how you will make your interpretation. In the example you just gave about how many times Peter should forgive his “brother,” context may determine who he meant when he was talking about his brother.
In that particular case, that question was not answered. Peter may have had his blood brother in mind or may have had a brother in the faith or a family member in mind. The passage doesn’t answer that specific question. Therefore since the bible is silent on this narrow question, I would contend it is a mistake to impose an answer anyway based on some outside source. A rule of interpretation is: be silent where the bible is silent and speak where the bible speaks.
As I mentioned on this site recently, Mark 6:3 and following is crucial to answering the “brother” question. Mark also list his four brothers and at least two sisters. But Jesus response refuted your idea that these four men were relatives or cousins. Jesus said, "A prophet is not without honor, except in his hometown and among his relatives, and his own household. (6:4)
Contained in Mark’s version are all of the variables in one place where Matthew leaves out the word “relatives.”
Jesus told us, he was not welcome in three distinct places: (1) His hometown, (2) among his relatives, AND… IN HIS OWN HOUSE."
This last part of the answer makes sense since the Greek word for brothers and sisters meant exactly that. a blood brother and sister. If everyone present were only “relatives,” the passage would have to read something like this:
“A prophet is not without honor, except in his hometown and among his RELATIVES who live in his own house.”
But that is not the case. A distinction is made between relatives AND, those of his own house. The implication is that those who lived in his house were the same ones listed by the men in the synagogue.
Secondly, I’ve heard the argument that the word firstborn doesn’t mean first of more sons born. But actually it does. A firstborn son, always receives a double portion of the inheritance. An only son is a different story. Matthew’s gospel was written well beyond the birth of Jesus’s brothers. He understood Jesus to be the first of at least four… This is the face value evidence found in scripture.
Reading a custom into the interpretation would be believable if the Greek counterparts were able to justify it. The holy Spirit did not use the Greek word for relatives or cousin when he said the word BROTHER. And, how this word is also used in other New Testament passages doesn’t surprise anyone. It means exactly what is says.
It’s time to get a grip to what inspired scripture actually says and hold it up higher than the traditions of men. This is what Jesus taught us to do.