Testing Lambda that requires other file

Trying to get a very simple Mocha and Sinon test running for a basic Lambda function. I have two files: a.js and b.js. I want to test the handler function in a.js

// a.js
const queue = require('../helpers/sqs')

module.exports.handler = async ev => {
  // do stuff
}

and

// b.js
const aws = require('aws-sdk')

...

As you can see, a requires b and b requires aws-sdk. I'm trying to get something very basic working but it's complaining that aws-sdk cannot be found. I want to test the handler function in a without b being called. How can I stub using Sinon so that the requirement for aws is not an issue?

const expect = require('chai').expect
const sinon = require('sinon')

const { handler } = require('../handlers/a')

describe('This', () => {

  it('should work', () => {
    expect(5).to.equal(5) // just something to work
  })
})