Hi, good question. This depends on whether the will (or trust) says the heirs or beneficiaries will take by "per stirpes" or "per capita."
Per stirpes means to take "by representation." Per capita means to take "by total number of individuals." If the taking is per stirpes, then the share that would have gone to the deceased sibling will pass to his or her children. If it's per capita, then the share that would have gone to the deceased sibling will be shared between the surviving siblings and the children of the deceased sibling would get nothing.