It is permissible for a wife to give her turn to a co-wife
حَدَّثَنَا أَبُو كُرَيْبٍ، مُحَمَّدُ بْنُ الْعَلاَءِ حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ كُنْتُ أَغَارُ عَلَى اللاَّتِي وَهَبْنَ أَنْفُسَهُنَّ لِرَسُولِ اللَّهِ صلى الله عليه وسلم وَأَقُولُ وَتَهَبُ الْمَرْأَةُ نَفْسَهَا فَلَمَّا أَنْزَلَ اللَّهُ عَزَّ وَجَلَّ { تُرْجِي مَنْ تَشَاءُ مِنْهُنَّ وَتُؤْوِي إِلَيْكَ مَنْ تَشَاءُ وَمَنِ ابْتَغَيْتَ مِمَّنْ عَزَلْتَ} قَالَتْ قُلْتُ وَاللَّهِ مَا أَرَى رَبَّكَ إِلاَّ يُسَارِعُ لَكَ فِي هَوَاكَ.
'A'isha (Allah be pleased with her) reported: I felt jealous of the women who offered themselves to Allah's Messenger (ﷺ) and said: Then when Allah, the Exalted and Glorious, revealed this:" You may defer any one of them you wish, and take to yourself any you wish; and if you desire any you have set aside (no sin is chargeable to you)" (xxxiii. 51), I ('A'isha.) said: It seems to me that your Lord hastens to satisfy your desire.