What should my older brothers and I do about our father's refusal to pay zakat and pray, and could sorcery be the cause of his change and deteriorating condition?

Your father is in great danger due to his neglect of prayer and zakat. You must advise him gently and remind him of their importance. Treat him kindly, associate with him in goodness, and seek help from those who have an influence on him. If symptoms of sorcery appear on him, then a lawful ruqyah is necessary, but the most important thing is to convince him to pray and pay zakat. Seek Allah's help through supplication for your father's guidance.