Why is my nested `.update` command running a joins on the background?

Question

I have a model:

class Brother < ActiveRecord
  has_many :sisters, through: :brothers_sisters
  has_many :brothers_sisters

  accepts_nested_attributes_for :sisters
end

And I would like to add a new Sister id: 3 to my @brother. I believe! I could write something like this :

@brother.update(sister_attributes: [{id: 1}, {id: 2}, {id: 3}])

However, this runs on the background this SQL :

  Brother Load (5.6ms)  SELECT `brothers`.* FROM `brothers` INNER JOIN `brothers_sisters` ON `brother`.`id` = `brothers_sisters`.`brother_id` WHERE `brothers_sisters`.`sister_id` = 1 AND `brothers`.`id` IN (1, 2, 3)

But because of the INNER JOIN, and because id: 3 does not exist, then the whole .update fails.

However if I run this, it works exactly as expected :

@brother.update(sister_ids: [1,2,3])

The Question: How can I accomplish auto-generating the missing joins tables but with the nested attributes in my first failing example?