Hey, Lamplight, and welcome to My Support Forums.
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I am an identical twin, now grown. I can tell you that our childhood emotional experiences were somewhat different. (My mother says she took me to look after since I was the smaller. On the other hand, as I got older, she admitted in retrospect that she treated me as a scapegoat.)
Twins differ in personalities, too, and that affects how we're treated. Yes, we were both given the same overall experiences, but in subtle ways we were treated differently, too.
Has your disgruntled daughter been willing to go into detail about how she thinks she was mistreated? Specific instances, things said, attitudes?
You mention friendships. Did they have different friends?
Maybe asking her to talk to you about things could help. Show her you care and feel bad that she views her childhood negatively. (Don't make her feel guilty , though, for being honest.) What did she perceive as differences in how she was treated? Perhaps a sincere apology is due, although she might say that it's too little, too late. But I hope not.
By the way, are your daughters emotionally stable or do they have issues that need to be addressed such as jealousy, anger, strong need for attention? And how old are they?
I hope I can be of help......
Sorry I was slow to answer. I hope you haven't given up on us.