The word "brother" is commonly used in peace treaties. Here are some Hittite-Egyptian examples (not exactly contemporary, but still useful):
Treaty between the Hittites and Egypt (1280 BCE) (ANET p. 199):
... while he is in brotherhood with me and he is at peace with me, and I am in brotherhood with him and I am at peace with him forever.
Treaty between Hattusilis and Ramses II (13th century) (ANET p. 202):
Treaty of Rea-mashesha mai Amana ... with Hattusilis ... his brother, for establishing [good] peace [and] good brotherhood... Now I have established good brotherhood (and) good peace between us forever.
When Moses tries to pass peacefully through Edom (Numbers 20:14), he speaks in the name of "your brother Israel." Amos (1:9) also adjures Tyre for forgetting the "covenant of brothers," connecting the two words.
I would guess that the language of "brotherhood" is used to give a sense of a treaty being between equals, rather than one side forcing it on the other. Since Aram became more powerful shortly after Ahab's death, he might not have had a choice in the matter.
In the specific context of peace between Israel and Aram, "he is my brother" might hearken back to the family relationship between Jacob and Laban (called his brother in Genesis 29:12,15, 31:46), who themselves made a treaty a long time earlier not to pass Gilead (Galed, or Jegar-sahadutha) in order to do wrong to one another (31:44-54).