Bismihi ta'ala

The expression used in this khutbah is:

 وَقَدْ نِلْتَ مَنْ صَهْرِهِ مَا لَمْ يَنَالاَ

and you also hold relationship to him by marriage which they do not hold.

There are different views regarding this, but what can most certainly be said is that the Prophet (s.a.w.) only had one living child, Seyyidah Fatimah al-Zahraa (a.s.). 

As for what this is referring to, the view is that 'Uthman was married to Ruqayah and Um Kulthum, the adopted or step-daughters of the Prophet (s.a.w.). The reason why I say adopted or step-daughters is that there is a strong view that Seyidah Khadijah (a.s.) was not married before the Prophet (s.a.w.), nor did she have any children. It is said they are the daughters of Seyidah Khadijah's sister Halah. 

Anyway, there are different opinions regarding this, and for further reading,  please visit this link [Arabic]:

https://hawzah.net/ar/Question/View/61641/%D8%B5%D8%AD%D8%A9-%D8%B2%D9%8...

And Allah knows best.