My sister is 88 years old & has dementia and cries every day all day long. because you're dealing with the mind, is there psychiatrists that deal primatily with dementia patients?

A number of sub specialists could be considered, such as a geriatrician, a neurologist, or a psychiatrist. There are behavioral neurologists who might be very helpful.

Yes, such behavioral changes are common in patients affected by dementia. Starting with her primary care doc would be best in trying to sort out her needs as many times somatic complaints, such as pain or general distress cannot be expressed properly by patients. In case needed, a referral will be placed in the community for a neurologist, geriatrician or psychiatrist.