James and John, not full biological brothers?

The late Roman Catholic scholar John Meier made a good point about the perpetual virginity of Mary that should be brought up more often. What's our initial impression when the terminology that's applied to Jesus is applied to other individuals? When the New Testament refers to James and John, the sons of Zebedee, as brothers, what's our initial impression about their relationship? That they're full biological siblings. Most likely, we retain that initial impression for the rest of our lives, unless we encounter overriding evidence. Terminology is sometimes applied in unusual ways. The term "son" can refer to an adoptive rather than biological relationship, for example, but that doesn't prevent us from recognizing that the biological meaning is more common. The New Testament qualifies Jesus' familial relationships with the virgin birth, but it never qualifies those relationships with something involving perpetual virginity on Mary's part. The absence of any effort to provide such a qualifier by so many authors across so many contexts is significant. My main point here, though, is that advocates of the perpetual virginity of Mary need to provide an overriding justification for interpreting the terminology the way they do. The way we interpret the relationship between James and John is an illustration of that.