Getting a child of child relation

I have three tables which are related.

A site has many patients which have many complaints. Here are abbreviated models.

class Site extends Model
{
    public function patients() 
    {
        return $this->hasMany('App\Patient');
    }
}

patient:

class Patient extends Model
{

    public function site()
    {
        return $this->belongsTo('App\Site');
    }

    public function complaints()
    {
        return $this->hasMany('App\Complaint');
    }
}

And finally, Complaint

class Complaint extends Model
{
    public function patient()
    {
        return $this->belongsTo('App\Patient');
    }

    public function site()
    {
        return $this->patient->site;
    }
}

So, my problem is that when I query complaints, I want to be able to get the site that the patient was in when that was generated.

Complaint::whereNull('resolved_at')->with('site')->get()

Gives me this:

Illuminate\Database\Eloquent\RelationNotFoundException with message 'Call to undefined relationship [site] on model [App\Complaint].'

One way to workaround was to add this to Complaint.php:

    protected $appends = ['site']; 

    public function getSiteAttribute()
    {
        return $this->patient->site;
    }

Which gives me this:

>>> Complaint::whereNull('resolved_at')->first()
=> App\Complaint {#211
     id: 1,
     reported_by: 7,
     patient_id: 4,
     problem: "Medicine made patient itchy",
     provider_id: 2,
     resolved_by: null,
// NOTE THAT site IS MISSING!!!
   }

Surprisingly, however, this does work:

>>> Complaint::whereNull('resolved_by')->first()->site
=> App\Site {#217
     id: 3,
     name: "Nursing Home 3 (pro)",
     site_info: null,
   }

So, what is the best way to make sure that site is listed in the reply?