It is permissible for a wife to give her turn to a co-wife Sahih Muslim 1463 b
Translation
A hadith like this has been transmitted on the authority of Hisham with the same chain of narrators (and the words are)
When Sauda became old (the rest of the hadith is the same) and in the narration of Sharik there is an addition (of these words:" She was the first woman whom he (Allah's Apostle) married after me."