U have every right to start feeling frustrated.
I think you are already giving her a lot by taking it in turns with your brother to keep her company. As for her freaking out when you came home a day later, even though she had company, it cannot be your problem.
I am assuming you are very much an adult and you and your siblings need to be allowed to live your lives again.
She needs to see a therapist to begin with. Try and convince her of this. She is battling to come to terms with the death, and needs some help to get on with life again