Step 1 : Introduction to the question "Your three-year-old daughter is acting jealous of her baby brother and is trying to push him away from your lap every time you're trying to feed the baby. What should you do?"
...Involving the older sibling in baby's care e.g. singing a song to the baby, helping to dry the baby after a bath, helping to change the diaper, etc. makes her feel important and not left out. Spend some time with her to play a game or do something she likes to do also helps reduce sibling rivalry. Reading a book to your child while feeding the baby is also helpful. Your child feels she can be close to you at the same time the baby is.