I don’t doubt for a second that Relena knew exactly what Mommy Darlian is talking about. Look at her face. She is absolutely desperate to believe that her father’s dying words were not truth. She is just praying that if she pretends it never happened, then it’s not true. She’ll just wipe that memory from her brain, and her mother will never bring it up, and they’ll just carry on as they had before.
But no. Her mother has something to tell her. She looks away and, again with the wonderful animation, she is shaking, literally shaking, with denial. She won’t be able to just pretend this never happened. She is going to have to face the truth.
And Mommy Darlian - she looks so resigned. No doubt she and Daddy Darlian discussed this many a late night, and decided when they would tell Relena she was adopted: either when she was old enough that she could take up the Peacecraft mantle and/or when Daddy Darlian was inevitably assassinated (It wouldn’t help anything to tell a five year old they’re heir to this big fancy political family). Evidently, Daddy Darlian recognized that Relena was ready now to take on the burden of The Last Peacecraft, and Mommy Darlian has reached the same conclusion. Their time protecting her is over. She is ready to face the very real danger of openly naming herself a Peacecraft.