The Messenger of Allah (P.B.U.H) did things in private, some of which may have been narrated by his wives.
By private, I mean within boundaries of his home.
Do these count as Sunnah ?
Yes it does. This is based on the following ayah:
وَاذْكُرْنَ مَا يُتْلَىٰ فِي بُيُوتِكُنَّ مِنْ آيَاتِ اللَّهِ وَالْحِكْمَةِ ۚ إِنَّ اللَّهَ كَانَ لَطِيفًا خَبِيرًا
[O Wives of the Prophet] Remember what is recited in your houses of God’s revelations and wisdom, for God is all subtle, all aware.
-Sura Al-Ahzab, Ayah 34
As this Ayah shows, the wives were commanded to remember (and by extension tell people) the wisdom which is the hadith and actions of the Prophet (SAW) even what he did in his house.
The Scholars of Hadith define sunnah as,
Anything narrated from or about the Prophet sallallaahu `alayhi wa sallam ( may Allah exalt his mention ) either before or after he became a prophet, of his statements, actions, confirmations, biography, and his physical character and attributes.
This includes all actions and sayings both private and public.
The private life of the Prophet sallallaahu `alayhi wa sallam has a lot of guidance for us, just like his public life.