Let's discuss the Requirement, My app has a child component which has to communicate with its sibling component. So it must happen through Parent with a Callback method from parent to one child which will get the value from child1 and will pass it to the other child as props. Simple, isnt it?

Req: Child1 has an input field, on typing that value must be sent to child component via Parent

First define a Callback function in Parent and send it as props to Child1

constructor(){
  super();
  this.state= {
    data : ""
  }
}

formChild1(params) {
  this.setState({
    data : params
  })
}

I have set a state variable data and updating its value using the callback function from child fromChild1 and sending the state value to Child2. Why using state ? Because props are Immutable and it is always better to pass state as props.

render() {
  return(
    <div>
      <p>Parent</p>
    <Child callback={this.formChild1.bind(this)} />
    <Child2 data={this.state.data} />
    </div>
  );
}

install prop-types because React.PropTypes is deprecated and React suggests you to use prop-types

npm install prop-types

Import prop-types in child1

import PropTypes from 'prop-types';

define child component

class Child1 extends Component {

getContent(event) {
this.props.callback(event.target.value);
}

    render() {
        return (
            <div>
                <p>Child One</p>
            <input type="text" onChange={this.getContent.bind(this)} placeholder="Type Something in Child One"/>
            </div>
        );
    }
}

getContent gets the input value onChange and will send it to parent through call back function.

Define prop-types for Child1 :

Child1.protoTypes = {
callback : PropTypes.func,
}

We have defined a function proptype for callback function.

Define a simple component which renders <p> tag with prop value sent to it from parent.

class Child2 extends Component {

render() {
    return(
        <p>Child2 : {this.props.data}</p>
    );
}

}

That's it. We have Now established a communication channel between sibling to sibling. And don't forget to export the child components if you have separate files for parent, child1 and child2. For reference see the code in /src/