Anonymous Asked: How to get my sister to stop crying?

so my little sister is 14 years old, 5 feet tall and 300 pounds. one day she came to me crying saying how she hated herself and how she wanted to lose weight, i just held her and told her id help her. our parents have never really been there for us so we have always had each other for support. i love my sister i dont like to see her sad, id do anything for my sister. i try to be the best big brother i can be, im 17 btw. im fit and i always work out (im 6'2" and 180 pounds) so i had figured i would work out with my sister to help her lose weight. the problem is every time things get hard for her she cries, and when she cries i end up going easy on her. last week was her first work out and we went jogging, but it was too much for her and she collapsed and had a hard time breathing. it was the scarriest thing i had ever seen. anyway, every work out she does i do with her to help encourage her and some of the work outs are really not that hard but she still cries, and when ever she cries its makes me feel bad, and i go easy on her. i want to help her, but i have to get her to stop crying. how can i do that?