Ah yes, the ears back is a difficult one. She's clearly VERY uncomfortable with him being near her.
However her body language is not really aggressive, she's pretty irritated, but not half as irritated as she is nervous.
She seems to lack confidence in herself, and feels as though she's being threatened in her household.
Trouble with kittens is that they REALLY don't read or display body language well at all!
This is what they learn through other cats as they grow up, and as they become adults.
In this picture, the fact that he is facing her front on and staring is quite intimidating to her,
that
and he is sat between her and a member of her family, your boy.
Try not to let your children (or yourself) be behind the newcomer, because this will be threatening to her.
But also not behind her, because she may feel that you also don't like this newcomer, and may lash out at him.
Try to remain on mutual ground, not favouring either cat.
When she calms down, you can begin to show her some great positive attention.
He's very curious isn't he! Not very bright though, bless him.
Persevere! It will be a bit iffy for a while, but see how it goes.
Don't forget to separate them tonight!
As mentioned somewhere in the thread, try feeding them breakfast in the same room, but on opposite sides.
Give it time. Go about your business in the room as normal.
I wish you the very best of luck!