How do you teach better hygiene to an 11 year old girl?

My sister has an 11 year old daughter. She recently has started puberty, but I do not think that she understands what is happening to her body. She has started her menstrual cycle and started getting body odor. My sister has attempted to talk to her on several occasions about bathing nightly and changing her Feminine products when they get dirty, but her daughter refuses to listen. She sometimes smells terrible and she allows blood to leak in to her pants. Could you please give me some advice to p on to my sister to help her, help her daughter understand why she needs to keep herself clean?