It is permissible for a wife to give her turn to a co-wife
حَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا جَرِيرٌ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ مَا رَأَيْتُ امْرَأَةً أَحَبَّ إِلَىَّ أَنْ أَكُونَ فِي مِسْلاَخِهَا مِنْ سَوْدَةَ بِنْتِ زَمْعَةَ مِنِ امْرَأَةٍ فِيهَا حِدَّةٌ قَالَتْ فَلَمَّا كَبِرَتْ جَعَلَتْ يَوْمَهَا مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم لِعَائِشَةَ قَالَتْ يَا رَسُولَ اللَّهِ قَدْ جَعَلْتُ يَوْمِي مِنْكَ لِعَائِشَةَ . فَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَقْسِمُ لِعَائِشَةَ يَوْمَيْنِ يَوْمَهَا وَيَوْمَ سَوْدَةَ .
'A'isha (Allah be pleased with her) reported: Never did I find any woman more loving to me than Sauda bint Zam'a. I wished I could be exactly like her who was passionate. As she became old, she had made over her day (which she had to spend) with Allah's Messenger (ﷺ) to 'A'isha. She said: I have made over my day with you to 'A'isha. So Allah's Messenger (ﷺ) allotted two days to 'A'isha, her own day (when it was her turn) and that of Sauda.