The Wives of the Prophet are not like Other Women
Allah says;
يَا نِسَاء النَّبِيِّ مَن يَأْتِ مِنكُنَّ بِفَاحِشَةٍ مُّبَيِّنَةٍ يُضَاعَفْ لَهَا الْعَذَابُ ضِعْفَيْنِ وَكَانَ ذَلِكَ عَلَى اللَّهِ يَسِيرًا
O wives of the Prophet! Whoever of you commits an open Fahishah, the torment for her will be doubled, and that is ever easy for Allah.
This Ayah is addressed to the wives of the Prophet who chose Allah and His Messenger and the Home of the Hereafter, and remained married to the Messenger of Allah. Thus it was befitting that there should be rulings which applied only to them, and not to other women, in the event that any of them should commit open Fahishah.
Ibn Abbas, may Allah be pleased with him, said;
"This means Nushuz (rebellion) and a bad attitude."
Whatever the case, this is a conditional phrase and it does not imply that what is referred to would actually happen. This is like the Ayat;
وَلَقَدْ أُوْحِىَ إِلَيْكَ وَإِلَى الَّذِينَ مِن قَبْلِكَ لَيِنْ أَشْرَكْتَ لَيَحْبَطَنَّ عَمَلُكَ
And indeed it has been revealed to you, as it was to those before you;"If you join others in worship with Allah, surely your deeds will be in vain." (39;65)
وَلَوْ أَشْرَكُواْ لَحَبِطَ عَنْهُمْ مَّا كَانُواْ يَعْمَلُونَ
But if they had joined in worship others with Allah, all that they used to do would have been of no benefit to them. (6;88)
قُلْ إِن كَانَ لِلرَّحْمَـنِ وَلَدٌ فَأَنَاْ أَوَّلُ الْعَـبِدِينَ
Say;"If the Most Gracious had a son, then I am the first of (Allah's) worshippers." (43;81)
لَّوْ أَرَادَ اللَّهُ أَن يَتَّخِذَ وَلَداً لاَّصْطَفَى مِمَّا يَخْلُقُ مَا يَشَأءُ سُبْحَـنَهُ هُوَ اللَّهُ الْوَحِدُ الْقَهَّارُ
Had Allah willed to take a son, He could have chosen whom He willed out of those whom He created. But glory be to Him! He is Allah, the One, the Irresistible. (39;4)
Because their status is so high, it is appropriate to state that the sin, if they were to commit it, would be so much worse, so as to protect them and their Hijab.
Allah says;
مَن يَأْتِ مِنكُنَّ بِفَاحِشَةٍ مُّبَيِّنَةٍ يُضَاعَفْ لَهَا الْعَذَابُ ضِعْفَيْنِ
Whoever of you commits an open Fahishah, the torment for her will be doubled,
Malik narrated from Zayd bin Aslam;
يُضَاعَفْ لَهَا الْعَذَابُ ضِعْفَيْنِ
(the torment for her will be doubled),
"In this world and the next."
Something similar was narrated from Ibn Abi Najih, from Mujahid.
وَكَانَ ذَلِكَ عَلَى اللَّهِ يَسِيرًا
and that is ever easy for Allah.
it is very easy indeed.
Then Allah mentions His justice and His bounty, in the Ayah