(1) Indeed, the Ritva writes that the Mishnah is Lo Zu Af Zu - not only is he believed on the father and Rebbi (not the son, incidentally), but even on the brother.

Ritva adds that since he is believed on his brother's writing, even though he is not so frequent with his brother nor frightened of him, it is likely that he is also believed on anyone's signature, because this is no different than the brother.

(2) I would like to suggest another reason why it is not sufficient to mention the brother alone, based on the Rosh's commentary. The Rosh disagrees with the Ritva and writes that he is only believed on his father's, teacher's or brother's signatures, but not on anyone else's. He cites the Yerushalmi which states this - that he is only believed on these 3 because he is familiar with them and continuously close to them, while he is never sufficiently familiar with anyone else (i.e. even though he is not as close to his brother as to the father and teacher, nevertheless he is closer to them than to other people, and the Rosh does not agree with the Ritva that the brother is just like the rest of the world).

According to the Rosh's explanation, I suggest that if the Mishnah would only have mentioned the brother, one would have said that since he is believed on his brother even though he is not extremely close to him, it follows that he is believed on everyone else too. Therefore the Mishnah specified father and Rabbi as well. The Rosh learns that since the Mishnah mentions the brother too, this teaches that there is a common factor between all 3 - namely that one is familiar with all of them. It follows that someone else apart from the 3 is not believed. Consequently it was necessary to mention all 3.

KOL TUV

Dovid Bloom