My hurting friend and her bulimic sis

Hey guys my best friend Allison came to me about her sis, who is also very dear to me. She found out her sis is bulimic. She has always been super worried about her weight. (Even tho it is fine an she looks great) she checks her weight bf and after every meal and hardly eats😢 Allison is like the sweetest person I know but she is super shy and doesn’t talk about much. When she tells me things I know its super important to her. She asked her sis what she was doing and she shrugged it off and denied it. But we can tell she deff is bulimic. Allison doesn’t know what to do and either do i! I want to help but idk how besides by praying. I want to tall her mom, (she is like a second mom to me so i have no prob talking to her) but Allison knows that her sis will be mad at her if she speaks up. Plz help me with any advice u can. I would ask my parents but I promised her I wouldn’t because my mom and her mom r close. Thanks guys!🙂💞😢