Yes they are childish but remember they are CHILDREN. They aren’t mature adults only the mc is. They are children with their own issue that they are struggling with give them time to grow and mature.
Yeah your so right and MC only just became mature due to the trauma of losing his family before that he was extremely selfish and childish as an adult!
And of course the little sister is going to cling to her big brother she lost her father so young and her brother stepped up to be the man of the house. So course as a child she's not going to react rationally to the though of him leaving her behind.
In my opinion I feel like James did not love her as a person but loved her as an object she was just something pretty for him to look at so he married her he knew nothing about her or cared to learn about her as a person she was just a novelty item for him to have a pretty wife that’s all she was to that selfish narcissistic prick that’s why he never saved her even though he had multiple chances to because he never cared about her as a person he was just satisfied to have the pretty woman he saw back at the academy as his wife I feel no remorse for that man at all he made his bed so better die in it