Useful Advice Needed Urgently: My Younger Sister Is Too Dull.

I had to bring this here maybe I could get some helpful advice here as I am really clueless about what to do about her. 

My younger sister is too dull. She is not smart and brilliant at all. She doesn’t love studying at all. She is doing so poor in school due to her lackadaisical attitude towards studying. I have tried to talk her and advice her into studying, but she will just looking through it as if she doesn’t know what she is reading and all. 

During the time she was doing WAEC and NECO, you won’t even know she was writting exam. My mum bought textbooks for her but she will end up misplacing them since she won’t read them. What she enjoy doing most is just to eat,sleep and all these children story books. She is now fat like Bleep. 

I always try to make her study or sometime even force her but there is little I can do for her as I can’t put all the book in her head and I am not even always around because I’m in school. 

I helped her check her waec result yesterday, and what I saw made me sad. She failed 6 subjects and passed only 3 subjects which she even had only C5 and C6. 

I don’t know what do to about her because I am tired. I don’t want her to gain admission into the university with this kind of spirit because it’s not going to help her at all. What can I do for her? How can I help her become more serious and become a better girl. She is 17. 

Useful Advices will be highly appreciated.
Mods, please do the needful.

Thanks