1)
Why did he call him "my brother"?
1.
Kli Yakar: It was to deceive him. I.e., even though the king made you the head general in place of me, I have no complaint against you, only against him. Therefore, Amasa did not guard himself from him.
2.
Perhaps it was because their mothers were sisters (17:25). Avraham told Lot "we are brothers" (Bereishis 13:8), even though Lot was his nephew. (PF)
