Because even if it was their own brother they had to be killed. A person might think that if it was their brother or good friend they may have mercy and spare them, therefore, it expressly writes 'brother' and 'fellow' to highlight the need for them all to be killed.
The Ibn Ezra writes:
והרגו איש את אחיו. אפי' יהיה אחיו. ורבים פירשו האומר לאביו ולאמו על זה המעשה והוצרכו לפרש אביו ואמו אבי האם ואין צורך כאשר אפרש במקומו:
AND SLAY EVERY MAN HIS BROTHER. Even if it be his brother. Many interpret Who said of his father, and of his mother: I have not seen him (Deut. 33:9Deut. 33:9) as referring to this incident. They had to explain his father and his mother as referring to the father of the mother. However, as I will explain, there is no need for this interpretation. (Sefaria translation)
And Ramban adds:
והרגו איש את אחיו שלא תחמלו ולא תכסו על אח וריע וקרוב
AND SLAY EVERY MAN HIS BROTHER. This means that they were not to spare nor conceal brother, friend, or relative.
An alternative approach is adopted by Rashi, who reads it more as a technicality, in that the term 'brother' here must imply that they share the same mother which would mean that they are not a Levi but a Yisrael instead, as all the Leviim were righteous. (Refer to Masechta Yoma 66b and also to the Sifsei Chachomim for his explanation on Rashi).