She still remained the wife of Baroka. In traditional polygamous families, a man wasn't required to divorce the eldest wife or some wives after marrying another woman. Of course, in the modern world a man can divorce the eldest wife and remain with the younger wives.
Before marrying Sidi, Baroka had two wives - Sidi the eldest and Favourite (her nickname) the youngest. Baroka just added Sidi in his list of wives.
The intention of Baroka marrying Sidi is not because she loved her but that she's was a threat to his authoritative power as the chief of Illunjire.
Lankule told Sadiku that she's is foolish by carrying out Baroka's request in convincing young women to get married to him. Also Sidi told Sadiku that she knows any woman who goes to Baroka's place either comes out of the place a concubine or a wife. Thus, there is a possibility Baroka has more than three wives.