My daughter and son's wife's rift is hurting the family

My eldest son’s romantic life has caused real issues in our family.

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He got married for the second time three years ago and before that he was with his first wife for 10 years.

The problem is, my daughter is still big friends with my son's first wife and his current wife can't accept it. My daughter regularly has nights out with the ex, which really upsets my daughter-in-law, who says it's weird. It results in my daughter and daughter-in-law not speaking to each other at all, which makes things extremely difficult for the family.

I find it so upsetting that we can't just be like everyone else and get together as a family to have a good time. This ridiculous standoff is ruining everything for everyone.