A bare bones shared state system using a pub/sub pattern and Reacts underlying useState hooks. See example in this Repl: https://repl.it/@cmlarsen/SharedState

StupidSimpleSharedState.js

import React, {useState, useEffect, useRef, useCallback, useMemo} from 'react' 
import * as ReactDOM from 'react-dom'
import {get, set} from 'lodash'

//The guts of the pub/sub system
const sharedState = {
  //Holds the state, this is mutable, but could easily be locked down
  state: {},
  //holds the watchers.
  watchers:{},
  //setup a new watcher for a "bit of state"
  watch (bitOfState, callback) {
    if (!this.watchers[bitOfState]) {
      this.watchers[bitOfState] = new Set();
    }
    //add the callback to the set of watchers
    this.watchers[bitOfState].add(callback)
    //fire the initial value
    callback(get(this.state, bitOfState))
  },
  //remote a watcher on a "bit of state"
  unwatch (bitOfState, callback) {
    this.watchers[bitOfState].delete(callback)
  },
  //set a value for a "bit of state"
  set(bitOfState, value) {
    //don't update if no change
    if (get(this.state, bitOfState) === value) return
    //update the state object
    set(this.state, bitOfState, value)
    //fan out the changes to all the watchers
    for (let callback of this.watchers[bitOfState]) {
      callback(get(this.state,bitOfState))
    }
  },
  //Initialize the state
  init(initialState){
    this.state = initialState
  }
}

//The main intercace hook
function useSharedState(bitOfState) {
  const [value, setValue] = useState(get(sharedState.state,bitOfState));
  
  useEffect(() => {
    const watcher = _value => {
      setValue(_value)
    }
    sharedState.watch(bitOfState, watcher)
    return () => {
      sharedState.unwatch(bitOfState, watcher)
    }
  }, [])
  
  const setter = useCallback((input) => {
    let value;
    if(typeof input === 'function'){
      value = input(get(this.state,bitOfState))
    } else {
      value = input
    }
     sharedState.set(bitOfState, input)
  },[])
  
  return [value, setter]
}


function ExampleComponent1(){
  const [time, setTime] = useSharedState('time')
  const [counter, setCounter] = useSharedState('nested.kinda.deep')
  const [weather, setWeather] = useSharedState('weather')
  

  useEffect(()=>{
    setInterval(()=>{
      //setting state based on current value
      setCounter(_value=>_value+1)
    },1000)
  },[])

  useEffect(()=>{
    fetch('https://api.weather.gov/gridpoints/TOP/31,80/forecast').then(res=>res.json()).then(result=>{
      setWeather(result.properties.periods[0].shortForecast)
    })
    
  },[]);

  return <div>Component 1 <br/> {counter} <br/> { time.toISOString()} <br/> {weather}  </div>
}

function ExampleComponent2(){
  const [time, setTime] = useSharedState('time')
  const [counter] = useSharedState('nested.kinda.deep')
  const [weather] = useSharedState('weather')

  useEffect(()=>{
    setInterval(()=>{
      setTime(new Date())
    },1000)
  },[])
 
 return <div>Component 2 <br/> {counter} <br/> { time.toISOString()} <br/> {weather}  </div>
}



function App(){
  sharedState.init({
    time:new Date(),
    weather:undefined,
    nested:{
      kinda:{
        deep:0
      }
    }})
    
    return <div className="App">
      <ExampleComponent1/>
      <ExampleComponent2/>
    </div> 
}

ReactDOM.render(<App />, document.getElementById('root'));

Could have problems if you subscribe to a child and then update it's parent.