You are right to be concerned. The fact that this is new behavior is a red flag that something is seriously wrong. He shouldn’t have to deteriorate more before something is done. You, and especially your little sister, shouldn’t have to put up with this.
You didn’t mention what your mother is doing in response to this behavior. If she is in the picture, she is probably in a better position to get your dad to the right help than you are. If she is not available, then please enlist the aid of someone your dad respects to help you take the steps that need to be taken.
He needs to see a doctor. He needs a complete physical check up to determine if there is an undiagnosed medical problem. A heart condition, a thyroid problem, even some vitamin deficiencies (to mention a few possibilities) can cause this kind of erratic behavior and irrational thinking.
If he is medically fine, then the family needs to get him to a mental health professional. I don’t have enough information to make a diagnosis, of course. But an interview with a professional will help determine whether there is a mental health issue.
Your father is a lucky man to have such a caring son. Trust your good instincts and do what you can to get him the help he needs.
I wish you well.