Unfortunately, your decision to terminate is a business decision that NEEDS to be made for your own health.
Who knows how long it will take things to settle down at home. I mean, they may need to play with her dosage etc... and although she is now on meds, that doesn't always fix bad habits/behaviour that they are doing at home to make it stressful etc. (Do you know what I mean?)
I'm not suggesting that we should not take this stuff into account, however, the fact that they were able to bring you to tears yesterday, shows just how stressed out you are with them here. I would not think you heartless at all if you went through with the termination.
If it would make you feel better why don't you set up a timeline for improvement and get mom on board, that way you gave them another chance, but with a definitive amount of time to see improvement.