Responses
I think it's cuz of what happen with their parents so he prob did it out of guilt, like not wanting his brother to go through the same fate as his mom and thought they would somehow be happy since the brother and princess kind of "liked" each other before she knew who he was.
This is kinda fucked up. Does the red haired brother of the ml (is he the current emperor or not, the translation is a bit confusing) actually care about his brother? Because his behaviour created unnecessary suffering and misunderstandings and did he just burn the letter before it could reach the ML? Looks to me like he didn't actually want things to work out between the FL and ML!