I got two kittens last year and they've always been very caring towards each other.
However, for the last few months, the older brother (who is physically bigger) has been chasing after the younger sister and doing a range of things like jumping on her and biting her (seemingly in a play fight manner), but she will always hiss, growl or even shriek in response. There haven't been any injuries and they do seem to mix it up in that the sister will chase after the brother on occasion.
Is this behaviour worrying? Is there something I should be doing to resolve this?
Do you have Feliway plugged in around the house?
A degree of play fighting is normal in cats though.