All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, may Allaah reward you for your keenness on being kind and dutiful to your parents and for being merciful to them.
The Sharee’ah has highlighted the right of being kind and dutiful to the parents in all cases even if they are wrong as this does not make them lose this right. The best way of being kind and dutiful to your parents is to endeavor to guide them to the straight path and teach them the matters of their religion. Also, you should supplicate Allaah to guide them and enable them to repent, and you should remind them of Allaah and the bad fate of the one who dies while he is negligent of his religion especially the prayer which is the pillar of Islam and the link between a person and his Lord.
For more benefit on how to be kind and dutiful to the parents and how to invite them to preserve the prayer, please refer to Fataawa 21794, 82254 and 19274.
Also, you should endeavor to eliminate the causes of this dissension between your parents as much as possible and reunite the family.
As regards cooking, then if they are able to do so, then it might be more appropriate to let them do it as this might be useful for them and they might feel at ease when serving their children. However, you should help them as much as possible even by purchasing food from restaurants sometimes and the like.
But if they, or one of them, cook with difficulty and they need help or a servant to help them and the like, then the children are obligated to serve them each according to his own ability. Ibn Muflih said in Al-Aadaab Ash-Shar’iyyah while clarifying the rights of the parents on their children: “Among their rights is to serve them if they both or one of them needs to be served, and honor their invitation and obey their order as long as it is not a sin.”
Allaah Knows best.