Your younger daughter needs to understand, as best she can at her age, the nature of her sister's disability. If she doesn't have a good understanding of the condition, she might find it hard to figure out why her sister is struggling so much to do what she can do so effortlessly. Without this information, she might make fun of her sister or feel sorry for her, neither of which will help. You might try reading a very good book about children with learning disabilities to both of the kids. It's called All Kinds of Minds, and it's written by Dr. Mel Levine.
Your daughter may find some comfort in knowing that many other kids have learning disabilities. If she can meet other students with LD it might help her understand that just because she is having difficulty reading, she's not bad or lazy or "stupid." If she's had a recent evaluation, ask the person who did the testing to explain the results to her in language that she can understand. You might want someone to do this every couple of years, as your daughter's capacity to understand increases with age. A learning disability is a life-long condition; the children who cope best are those who understand what it is and how to be successful, even though they have a learning disability.