In my opinion, Sadiku is partly to be blamed for the turn of events in the book. She rejoiced in the fact her husband, Baroka was impotent. She couldn't contain her joy by gossiping this fact, which we later learn was a lie from Baroka, to Sidi.
Sidi saw an opportunity of mocking Baroka, indirectly, pertaining to his impotence not realizing Baroka deceived his elder wife, Sadiku that he's impotent.
If Sadiku had kept quiet about it, Sidi would not have been deflowered. She would not have ended as Baroka's next wife.
As noted above, I can't fully blame her for the turn of events. Sidi is also to be blamed. She knew Baroka was cunning. Before the incident of Sidi ending up in Baroka's arms, she had told Sadiku that she knew Baroka very well. She knew every woman who visited Baroka at his place would end up as a wife or concubine. If she knew Baroka's cunning nature, why did she go to his place to mock him? Wasn't she slanderous? Didn't she enjoy demeaning people? Wasn't it as a result of her pride because she was the most beautiful girl in the village?