Your headline is incorrect. Your DIL is not trying to ruin your relationship with your DD. Your DD has ruined your relationship with your DS, as have you, because you are an enabler.
Your DD didn’t take to DIL. You mean she didn’t like her. Why? Was it jealousy? Your DD took the piss out of her in front of you. She did it whilst you watched. She was being her. What does that mean exactly? Taking the piss out of her accent, class, mannerisms? What did you do? Stand there and watch, laugh?
Your DIL heard it, and would have been really upset. She was probably really hurt, embarrassed and felt bullied. I’m surprised their relationship survived.
Then, your DD did nothing. She didn’t apologise for her lack of respect and duplicity. Whats more your DD is claiming to be the victim here, and you are backing her? Why?
At this stage my own parent would’ve taken me aside and had a serious word with me about my behaviour, and I would’ve apologised profusely and meant it. If that didn’t work I’d have written her a letter and took steps to make it up to her. Your DIL is still waiting for an apology 4 years later.
I am part of a similar set up. A badly behaved, spoilt jealous SIL who bad mouths me behind my back and a MIL who thinks she can do no wrong. If I ever hear it first hand, I’ll go NC.
You are just another one of those mums who has a DD and. Son and backs her DD no matter what because you think she’s your insurance policy.
DH and I are the only ones to have DC, and DGC will be along soon. As a younger woman I sucked it up. As a mother, and a mature woman they can go whistle. I see them once a year and my DC give no shits that they are relatives.