Assalaamu alaykum. I would like to know what a person should do when they are dealing with a toxic person/mentally sick person? Did Prophet Muhammad said anything on how we should treat these individuals when they are interacting with others? I have an aunt who is quite toxic, and she often picks fights with other family members. They all tried their best to be quiet and let her say whatever, but in the end they all lose their patience and temper and end up raising their voice and talking back at her.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

This is a kind of affliction which you should meet with patience, especially given that she is your aunt and your relative.

If she is excused and is no longer held accountable for her deeds (i.e. not competent for religious assignments), then it is a greater reason for being patient with her and supplicate Allah to cure her.

For more benefit on the virtue of patience, please refer to fatwa 83577.

In regard to what is reported in the Sunnah about the injustice of relatives and how to deal with them is what was reported by Muslim and narrated by Abu Hurayrah, may Allah be pleased with him, that a man said to the Prophet “O! Messenger of Allah, I have relatives with whom I try to keep the ties of kinship but they sever relations with me; and whom I treat kindly but they treat me badly; I am gentle with them but they are rough with me.” Thereupon, the Prophet replied, “If you are as you say, it is as if you are feeding them hot ashes, and you will have a supporter against them from Allah as long as you continue to do so.”

Nonetheless, if your aunt is sane, then you should advise her in a kind and gentle manner and clarify the bad fate of her actions in this world and the Hereafter to her. Her mental state is not an excuse in front of Allah as long as she is sane.

Hence, you may seek the help of someone whom she is afraid of and whom you think she may listen to; then if she regains her senses, then this is what is required, but if she continues in her ill conduct, then you may desert her if you believe that this will be beneficial to her. However, if you think that this will make her even more stubborn, then you should not desert her because deserting a person depends on the benefit, as has been stated by the scholars. For more benefit, please refer to fatwa 347560.

Allah knows best.