Is it permissible for a girl to pray in front of her brothers and father?

There is no harm for a woman to pray in the presence of one of her mahrams, as long as she covers what is obligatory for her to cover in prayer.

Sheikh Abdul Aziz bin Baz, may Allah have mercy on him, was asked about the permissibility of a woman praying while a mahram (definition) man is behind her, whether asleep or awake. He replied: "There is no harm in that. There is no harm for her to pray whether he is in front of her or behind her. However, it does not matter if he is sitting or lying down. But if his face is towards her, it is preferable not to do so. She should turn away from him like this or like that, because he might distract her if his face is in front of her, or if a woman's face is in front of her. But if he has his back to her, or his side, then there is no harm in that, or if he is lying down, there is no harm in that."