I'm very sorry about your father's dementia. As Kathy said, you're really fortunate to have another family member for support.
If your father has been diagnosed with Alzheimer's, is he taking any medication for it? Have you spoken to his doctor about his outbursts? There are medications that may help with that, too. It's a fairly normal part of Alzheimer's -- and probably other dementias -- but it's very distressing, especially if it gets bad enough to make people afraid of him.
If he is unable to handle his affairs and unwilling to sign the necessary documents, an attorney can help you. It would be good to find one who specializes in elder law. It doesn't need to be horribly expensive, but definitely needs to be done.
If you haven't been to the
Alzheimer's Organization site, please check it out. There is an enormous amount of information there. There's a section on financial and legal matters that you may find especially helpful right now. A local Alzheimer's support group can also make going through this a little easier.
As far as feeling agitated with your father, please don't feel guilty. You're in a terribly stressful situation and it's completely normal to feel frustrated sometimes. No matter how much you try to remember that the frustrations are caused by the disease and not willful stubbornness by your father, sometimes things just bubble over. It's one of the reasons a support group can make such a difference. Everyone loses patience now and then -- it's just part of our human nature.
You are being a kind and loving child, and your father is fortunate to have you to care for him when he needs you.
Best wishes-
Carol