Mine ex was like this with our oldest daughter. He always expected too much from her and even tho she was the sensible one, he'd always move the goal posts. I used to hate leaving him with the both of them. Yet DS was a much harder kid to handle, but he was far more tolerant with him.
My ex had an older sister that got his parents attention continually (was put on a pedestal)and I often used to think he didn't like 'females' because of this.
Move the clocks on about 15 years - we are divorced - his attitude towards her got worse despite the fact that she is a wonderful daughter - and they now, today, don't speak at all and she loathes him.
He always accused her of being devious and manipulating, being my precious one (which wasn't true), I could see the frostiness with him for her.
I suppose I didn't handle it very well, and she often recalls things that I would have handled differently today. Plus when you are in the thick of it you can't quite see it. Alarm bells of unfairness would go ogg and I would let it ride for a quiet life. But today, she recalls it, nothing is forgotten and she has a pile of resentment towards him. Her self esteem is bruised and she chose all the wrong men for the 'negative confort zone' issue. Address this problem.
I also think - her being first-born - he somehow resented her coming along and taking away our 'together you and I' thing. He was ok when she was a baby, he was patient and besotted, but the more she grew into her personality, the more he distanced himself.
I remember looking at a old photo where he has DS on his lap kissing his forehead and she is standing beside 'left out' and it seems to summarise what she felt like in all aspects of whatever she did with him.
This was one of the reasons we fell apart, I wish I had done something beforehand and saved her the destruction it created in her self image.
I had a wonderful father and was idolised by him, I couldn't identify with what I experiencing regarding their relationship. I know now, not all Dads are like mine was.
He wasn't horrible to her, but just unfair, continually. He always expected her to be the respinsible one, the compliant one, the one who would 'give-in' for the sake of an easy life (for him reagrding care of the two children).
Nip this in the bud, but without creating a black hole. It is so easily interpreted as you being over protective or 'siding' with her. Don't let it happen. I did, without realising, and today she is the one who suffers. Not me, I'm out of it, but that is her first relationship where she learns about a mans love and it has to be pointed out to your husband how important his input is, with her.
Bit long this response, but it struck something in me when I read it.