(This is up to chapter 24!!)
The duchess was also glaring at the aunt, while also trembling. Possibly because she is reborn too and knows what the aunt did to Luca. Duchess also overly questioned the aunt, possibly because she knows the aunt is lying about being the mother.
Luka also reacted to the assassin on the train pre-emptively, and against the assassin and when being taught + reading. He is too skilled/experienced. Has he already experienced a train and snow before? experienced that dessert in poverty as well?
But then he seemed genuinely upset that that aunt was at danger, either:
A) he knew the assassin was there, but did not think aunt would put herself over himself.
B) He forgot /does not know about the assassin, but his experience from a past life allowed him to pick up odd signs from the assassin.
Also, Luca did say he was not a child, admittedly a child would say that, but so would an adult who feels embarrassed acting like a child etc.

I'm getting that feeling, I'm on ch 12 and he said to the uncle "You weren't like this before", about his flirting with Eudith