Working remotely for a large bank in Ontario for 10+ years with high performance, if I begin to miss meetings and my work quality and quantity significantly goes down due to personal family life circumstances (not medical), if my employer chooses to fire me, would they be required to pay severance? And would they be required to give me official warnings prior to dismissal? Thank you.

If your work quality and attendance are suffering due to family life circumstances, you should advise your employer of the family life circumstances as soon as possible so that your employer can accommodate any code related requirements.
Once your employer is aware of the cause of any potential performance issues, they must accommodate your condition up to the point of undue hardship. Your employer cannot fire you because of this.
It is strongly recommended that you schedule a consultation with a lawyer to discuss your situation in more detail and provide you with guidance on how to best approach this situation and what your options are going forward. Please contact our intake department to schedule a consultation as soon as possible.