It Takes a Village to Raise a Child
Recently I got an email from a parent who's child was not talking as quickly as his older brother had talked at this age. She was frustrated because when she asks her toddler a question, he often looks to his older sibling to answer for him. This is not an uncommon occurence. Children in the same family often learn speech at different rates. Although we don't always know the reason for speech delays, birth order often plays a role in speech development.
Younger siblings naturally look up to their older siblings. As the youngest of four, I always wanted to be like my big sisters, whether they were doing the right thing or not. If they said, "The sky is purple!" I believed it. I often took their advice over my parents.
If this sounds similar to your family, then think about involving the siblings in helping your child talk. Use the siblings to model correct speech or help introduce new signs.
However, there needs to be clear boundaries for the siblings.
If you are doing a speech activity, the older siblings can not talk for your child. During a speech activity, is a great opportunity to make use of siblings' help. Let both of your children play the game or do the activity. Your older child will probably enjoy being the teacher. You can use a prop, such as a toy microphone or "talking turtle" puppet, as a fun way to let your child know when it is his or her turn to talk. If the older sibling answers for his younger sibling, simply say, "Oops! Ashton has the microphone now. It's his turn to talk."
If your child is requesting a want or need, the older siblings can not do the talking. Here's a scenario. Caleb wants wants a ball that is out of reach. When he points to it, his sister Emily says, "Mom, he wants a ball!" You can simply answer, "It's Caleb's turn to talk...Caleb, if you want the ball, say (or sign), 'ball'." If he refuses to try, then you don't give him the ball.
So, if your child is naturally looking up to his or her sibling, don't see it as a nuisance, see it as a tool.