DD has been hitting. She was sent to cool down.
Later DS1 hit her and she came up crying. I went down and DS1 was playing and grinning in the garden. I sent him in.
Asked DH why he didn't do anything and he said they were both as bad as each other.
Not disciplining ds1 for hitting dd might teach dd that both are as bad as each other but ds1 would feel he got away with it, maybe.
Ds1 is now full of attitude, dd is okay and I am fed up.