I think it was the realization that thats what happened in her first life. She previously stated Morris just left and became a mercenary, but was unsure why. This would give a good connection for why it would have happened (cuz her brother likely tried to kill Morris to keep him from inheriting and he defended himself) and why he went away from inheriting the thing he never wanted in the first place.
Makes me wonder if his brother was told something different or doesn't know imperial law like Sigrid does.