I don't think it will hurt either of them to watch the extra lessons. Especially if the oldest isn't reading yet, she could benefit from the input! My youngest loves having older sister act things out and point to the words while we do LR. It helps the older one feel important if they get to be the teacher. Maybe show your older one how to point to the words, repeating each one so baby can learn it. She may learn to read in the meantime! I have found that the more things they do together as a team (even though mine are 3&1/2 years apart), the better they get along. They love each other so much because there is no competition between them, just team work! Eventually, the younger one starts wanting to copy the older one just because she sees how much fun Sister is having.
thank for your reply, the older one is not reading yet, although she recognizes couple of words from the books when we read to her, but she is reluctant to read by herself or even look at books. Now I am just showing either of their lessons whenever they are both around, will see how it works out :-)