My husband and I often cringe at the way our dil occasionally talks to our son. We have learned not to comment and hopefully she gets the message that we do not like it and will not get involved. She can be moody and judgemental, but nice and supposedly caring at others, but we do wonder if at these times she is merely saying what she thinks we would like to hear. Her opinions change! We remain neutral but if our son does have a word on his own we tell him he needs to be more assertive, but never criticise dil.
We are not confrontational and our children have grown up in a supportive family where we will always be there for them, to help out practically if they want us. Our son works hard and is a fantastic father, and certainly does his bit around the house but, like his father, does not like arguments and perhaps needs to be more assertive and have stronger opinions. However, the couple have to work things out for themselves. They knew what each was like before they married and it is early days.