It's so hard to lose weight?

My mom has lung and bone cancer, and it makes it hard emotionally . She eats a lot a few days after chemotherapy. She brings junk in the house. That combined with stress it too much... My dad is also always depressed. Im not blaming this on them. I'm 15, female and 200 pounds. I need to lose weight . I know how but it seems there is so much in my way! Ps 'my sister lost 20 pounds easily , why can't I ?