Nsryjdtyk

I've created a model (see Delegation) made of basic attributes (name, activity) and one collection (members).

See jsfiddle

The fetch method update the model and trigger a sync event on it - but I would like to be notified when the embedded collection is synced (ie in a real case to render a view when the inner collection is synced).

I tried this

this.listenTo(this.get('members'),'sync',function(){...}

but the corresponding event never fires.

What is the proper way to have it triggered?

When you execute the fetch function of Backbone, it returns a
PROMISE, you can use the function then to launch a function at the
moment the fetch is finished, you can also use the catch for when
it fails.

For example:

this.model = new Backbone.Model()

this.model.url ="example"

this.model.fetch().then((data) => { 

    console.log("succes: " + data); // Finished the fetch successfully 

  }).catch((data) =>{ 

    console.log("error: " + data); // The fetch ended in error

  })

For your code:

You can use it in this part Backbone.Model.prototype.fetch or if your fetch function returns a Promise, use it when calling the fetch of the model.

I don't see any code that fetches the collection, hence sync is not triggered on the collection.
You should do delegation1.get('members').fetch(). For this to work, the collection should have it's own URL as well.