If Matthew and the other authors considered cousins bros, I wonder why he didn't refer to John the Baptist as "brother John"? ...another reason for the plus.
1. The recorded speech of one of them to the other tends to be coached in lofty terms: "I must decrease for He must increase"; "so it becometh us to fulfill all justice". This precluded "brother" as an affectionate term.
2. The expansive use of "brother" is common when several relatives are addressed collectively. When just one person is addressed, a more precise term, such as "cousin" would be typically used.
3. John was renowned in his own right, while James a.o. were only known in relation to Jesus.