The terms for brother, step-brother, and cousin did not have the distinct meanings that they have in modern English, hence the misinterpretations that result from using modern English as the definitive basis for one's interpretations.
Classical Hebrew has a documented vocabulary of about 25,000 words. This is approximately 1/40th the size of the current English vocabulary. The case of classical Greek is similar.
Even discounting the explosion of technological terms in the past couple centuries, words in classical languages almost always had a wider and less precise range of meanings than we expect from modern English usage.
To squint through 21st century spectacles at a seventeenth century translation is a conspicuously error-prone method of determining the meaning of difficult parts of God’s word.