How to deal with kids mean comments

How would you guys deal with a situation where an older child (5 years old) said mean things to a younger child (18 month old)? Like call them stupid and saying they don’t like them. It usually occurs when the older child is upset about something completely unrelated to the younger child (like if he’s told to put his coat on or he’s been given something for lunch he doesn’t want, etc.).

The younger child is my own child and the older child is my nephew.