My sister Has this problem??! Please Help and read more?

My sister is 5 and i am 15. it started when i was sat with me sister on my lap as she was crying after she fell over. And then she said i need to go PEEE PEEE. She had fallen over and hurt and really badly grazed her knee so my dad asked me if i could carry her to the toilet. And this obviously took to long as half way up the stairs she starts wailing and i look down and she has peed all over herself and me. We bathed her and then i took a shower. Then we took her too the doctors for her knee and they said that it was badly sprained and also had a chipped bone. They didnt want to plaster it so we just carried her around. But then after it healed, she had got too lazy and asked to be carried around everywhere. Me, my mom, my dad and 12 year old brother started to get a bit reluctent to do it. Then one day, she was sat in the garden and my brother was off school as i had exams and she just said CHELSEY I NEED TO GO POOP and i said well go then and she said she didnt need to go anymore. My mom and dad were inside and then 5 minutes later she runs up the stairs into her room and she has pooped herself. So we bathed her again. But now, she wont use the toilet or feed herself. She is very lazy and everytime she pees or poops herself we have to clean her up so she doesnt get infections. She does ot at school too. What shall we do as a family?