No comentário do @SS-3 talvez seja outra maneira de responder a pergunta.
Edit: 1) material adicional de apoio (re: Kylo Ren) do romance foi adicionado para fornecer uma ilustração mais abrangente da 'ação e reação' bidirecional entre Kylo Ren e Leia. 2) O uso de bonés foi reduzido.
1) Leia sabe que Han tentará
'reach' their son.
Isso é um fato, já que isso é algo que ela especificamente pede a Han antes de ele ir embora, como ela diz a ele para
"bring him home".
2) Aqueles sensíveis à Força têm o potencial de sentir outros que também são sensíveis à Força ... (ou sentem a morte de muitos como com a destruição de Alderaan, por exemplo).
This is a fact re-suggested in the movie when the Supreme Leader and Kylo Ren talk about sensing Rey who is sensitive to the Force and is awakening to it.
Com essas duas ideias em mente, minha resposta é que
Force-sensitive Leia sensed the abomination that Kylo Ren - her own force-sensitive son - did, which was to kill his own father.
Acredito que essa resposta seja potencialmente mais apoiada por duas instâncias específicas do livro em que:
1a) Kylo Ren reage à menção de sua mãe ...
“No, it’s not.” Halfway across the walkway now, Han continued to move forward, smiling. “Never too late for the truth. Leave here with me. Come home.” Without the slightest trace of malice or deception, he cast a dagger. “Your mother misses you.”
A strange sensation touched the younger man’s cheeks. Something long forgotten. Dampness. Tears.
“I’m being torn apart. I want—I want to be free of this pain.”
1b) Kylo Ren reage ao que ele fez ...
Stunned by his own action, Kylo Ren fell to his knees. Following through on the act ought to have made him stronger, a part of him believed. Instead, he found himself weakened.
e
2) Leia admite que ela sabia de
their son's potential corruption by and to the Dark side since the beginning... and yet tried to deny it and hide it because 1) she didn't think Han would handle it too well, and 2) she wanted to believe that their son could be kept with the Light and either not turned or capable of redemption.
Neste, Leia mostra
denial - YEARS of denial - when in truth, she knew deep down and long ago what really was going on and was likely to happen.
Sending Han out to try and save their son reads, to me, like another last-ditch effort where she - once again - is 'naively' hoping for something that will likely not happen the way she wishes to.
Embora ainda não tenhamos ideia se
redemption is in the cards for Ben, killing his own father
é um grande negócio e aliado ao fato de que
Kylo Ren is shown to emotionally react to the mention of his mother (or just the sense of family)...
Para Leia, eu acho que isso é mais do que o suficiente de um 'distúrbio' na Força
(specifically, her son emotionally reacting towards the mention of her and family and being corrupted/turned enough to the Dark to kill his own father)
para ela sentir e estar ciente.
Nota: Por favor, desculpe o formato instável desta resposta, pois estou tentando o meu melhor para garantir que a maioria do material digno de spoiler seja mantido atrás das tags do spoiler, mantendo a minha resposta legível.