I'm pretty sure it's because the younger brother was hiding it from his mom at that point (she thought they were friends). Xia Chen on the other hand felt bad about his mother finding out; so he stopped his relationship with Lu Feng. However, the younger brother didn't go through that type of trauma, so he was able to hold a relationship secretly without feeling as bad as Xia Chen (though I am sure he felt bad overall). It's sad to me that they felt bad in the first place, but it's something that a lot of people go through. So yeah! While Xia Chen does date Lu Feng in secret after they meet again, he ultimately chooses his mother, (looking at it from the point before Lu Feng does what he does in the end) while his brother chose to stay in a secret relationship.
