I think it shows appalling manners to 'univite' someone from a wedding, especially the wife of the bride's father.
Admittedly, this is only 'news' because Sian Lloyd is on television, but even so, a degree of protocol needs to be observed.
From my experience of weddings - my own, and three daughters' - the words 'marriage' and 'trouble' go together like the proverbial horse-and-carriage. You cannot escape issues, only hope to minimise them, and try to ensure that the day runs smoothly.
To 'uninvite' a prominent guest is not the way to go about minimising problems.
For the sake of the couple, whom I assume are in agreement with Ms Lloyd's attendance, the bride's parents should have sorted their differences at another time. making a point to an unwanted guest in this way is not the way towards a smooth-running happy memorable day for the bride and groom, and they are the ones who matter in all of this - squabbling in-laws can park their differences.