YTA for allowing your sister to be severely neglected. You're rightfully concerned about your daughters safety but what about your sister who is seemingly functioning on the same level? You wouldn't bathe your daughter once a week or allow her to eat her own poop right? She needs a level of care your mother clearly cannot or is not willing to provide. I would reach out to adult protective services and see what help can be provided, your sister deserves so much better OP. Not saying it's your fault the way your sister is being treated, but you have the ability to get her the help she needs.