Biting my tongue was something I had to learn where DGs were concerned.
What works for us is I accept DD has ways of doing things in her home with her children but both DGs know some rules are slightly different at Granny's, which they adhere to.
I have though sometimes forgotten my own rule and but in before I realise it, smile at DD and say oops but at the same time, the girls have also been reminded to mind their manners/stop bickering.
Re any couples relationship. I stay well clear as I don't know the full story.