Read v. 35, "For whosoever shall do the will of God, he is my brother, and my sister, and mother."
not in this passage, or John 7. In this passage, the distinction is made between his bros, as in related and close to Mary and all others.
In John 7, I agree, the likely reference is to relatives.
The term may apply to cousin.
OK, but I don't think Jesus referred to John the Baptist as a relative in any particular way. Why? I offered some ideas, but the simple answer is, we don't know.
John was a prophet. That was their notable relationship. It really does't mean anything if one is simply Jesus' brother, or cousin. Even the reference to them(and Mary) in Mark 3 was only as an introduction to His teaching.