Ssdi question

Hi I have a question? My sister is applying for disability and cause she doesn't have enough work credits and isn't 62 or older . She only qualify under our parents social security, my mother receives cause of her retirement age. And my father received his due to him being disabled . So my question is they are trying to go off of there social security. How does that work when it comes to how much she would receive. Both parents are divorced too . So would they go off of both my parents or just who ever receives the most each month