'She was of different tribe' was not a reason for Nnaemeka’s objection to the girl his father has chosen.
Nnaemeka didn't want to the girl his father Okeke has chosen for him. He loved Nene and wanted to marry her. Okeke was against the marriage as Nene belonged to a tribe. Okeke wanted his son to marry the girl of the same tribe in order to get acceptance from the society. Since he was one of the renowned people in his village, he didn't want his son to commit to anything which would turn his position in front of the village.