The doctor put my mother on two ssri's. Lexapro (escitalopram) 20 mg and added Zoloft 50 mg (in place of abilify). Is this safe? I worry about seratonin syndrome.

It depends. It depends on what the doctor is trying to accomplish -- which you & your mother can ask him/her about. This is a low dose of zoloft (sertraline), (sertraline) and s/he could be cross-tapering onto a new medication. Yes 2 ssri's at same time could lead to serotonin syndrome, but probably not at the low dose of zoloft (sertraline) you list. Please ask your mother's physician about your concerns.
Talk with the doctor. You may want to discuss with your mother's permission your concerns with the doctor. My clinical intuition is that it should be okay although if it was felt that there might be a role for an even more cautious approach then one could envision a scenario where sertraline 25 mg is used initially instead of 50 mg. There could even be an initial implementation of 12.5 mg. Stay safe!
Reasons to question. I read above that your age is 68. I can only surmise that your mother is quite elderly. I am not sure there is much to gain adding zoloft (sertraline) to another higher-dosed ssri, and you have good reasons to worry about serotonin syndrome. Even moreso at her age. I agree that, with her permission, you should talk to her doctor about this.