Is 12 too late to toilet train? + Asking for book reccs

My youngest sister is 12 with down syndrome and autism.

She can feed herself, but is non verbal (she vocalises but doesn't speak). She understands when we ask her for certain things like to go shower and to have her dinner. But she is not toilet trained, and wears nappies. Is it still possible at this age? I don't live at home but I want to be more involved and am going to visit every month or so for a couple of weeks at a time.

While I'm here I was also curious if anyone had any good book recommendations for a situation like hers? Not just the toilet training but in general for strategies to help manage.

Thanks in advance.