If their happiness is lessened by not ostracizing a member of the family, that is their problem. Its all nonsense. If one family is wearing a certain color, all family members should be included. The daughter is trying to exclude one family member, basically saying, you are not a part of my family, so you should not dress like the rest of us. Its inappropriate.
I agree with being hands off with unimportant details. But the daughter invited her step-sister to the wedding, and is now saying you cannot "be a part of our family." Its just wrong. I would tell my daughter to be respectful of my new wife and her kids.
Its not a matter of wedding particulars. Its a matter of respect.
Totally agree with this. the daughter is entitled to her day, she's not entitled to make people feel ostracized or ridiculous.
Grow some balls OP and tell your daughter, that all your wifes daughters are wearing the same colors, and thats the end of that.