Hi Thaila!
So, when my brother was 4, they started teaching reading at school. My brother loved stories, but he was rather finding reading difficult. And what I did was, whenever my brother comes with a story book to me, I read it, and ask him to read few easy beginner words. This happened all the time we read, and my brother could soon read most of the words in Famous Five. I kept helping him in many ways. For instance, when we play a school game, he would be the teacher and me, the student. I would occassionaly ask him to read words like - CAT, TIN, FISH, GUN, TOY, BEN, TOM, ect. He gradually improved and became wonderful. My brother is 5 years old now, and he can read more than half the words in Famous Five (they haven't thought the words bigger than 'because' yet). And if it's an easy book, he can read the whole book without any help. If your sister likes animals and birds, give her easy Animal Books. If it is superheroes, give her small beginner's superhero books.