I'll assume your sister's mental condition isn't disqualifying (a mental disorder and behavior associated with the disorder that may pose, or has posed, a threat to the property, safety, or welfare of the alien or others). The issue is that for your sponsorship she would be in fourth visa preference category, and right now we're talking about a wait of approximately 11 years for the approval of family members in that category. (See the Visa Bulletin.) If your father sponsors her as an adult child over the age of 21, that puts her in category 2b which involves a @8 year wait.

