Is one obligated to help a sick father who previously neglected his son, or are the sons more entitled to the money, and is not helping considered haram?

No matter how negligent a father may be, his right to dutifulness and companionship with kindness does not fall away; due to the Almighty's saying: "And accompany them in the world with kindness." If he is in need of financial support, it is obligatory for his affluent children to spend on him. If all of them refuse, and you have a surplus beyond your needs and your family's needs, then it is obligatory for you to spend on him. If he is not poor but asks you for money, and you are able to give it to him without harm, then do not hesitate. For dutifulness to parents and showing kindness to them are among the best deeds and the most beloved to Allah. In the Hadith: "The Lord's pleasure is in the parent's pleasure, and the Lord's displeasure is in the parent's displeasure," and "The parent is the middle of the gates of Paradise."