M., this is a very difficult question to ask... but because of my personal experience, I'm going to go ahead and ask it.
Is there any possibility that the girls perceive any favoritism on their father's or your part?
Whew. TOld you it would be a tough question.
You may be thinking, "WHAT??? I"m a GOOD mom! I don't favor one child over another!!!!" ANd please don't think I was insulting you. It is not possible for two people to have exactly the same personality, so we react differently to each of our children.
If one is usually chipper and happy, we tend to respond more lightheartedly. If the other is more introspective and serious, we tend to mirror that behavior to them. (Doing otherwise might make them feel as if we didn't take their thoughts seriously.)
But-- when the two kinds of children analyze the parent's reaction without taking into account their own personalities, they can view your lightheartedness with one as "favoritism."
That perception of favoritism will create feelings of animosity between the two-- no doubt about it! (Just look at Jacob & Esau, Leah & Rachel, Cain & Abel!!!!)
It's simple to rebalance that perception-- simply choose to spend a little more time with the more-serious one, talking about their interests, etc. (You'll have to fight through their teenage lack of desire to share, possibly! LOL!!!)
another thought-- "not hearing" the behavior doesn't mean it isn't happening." Be sure to phrase your request so they don't hear, "As long as I don't know about it, it's ok!"
Hope this helps!!!