[Oh, he's not implying anything. Only making a casual little observation, in all innocence, about how the big brother apparently can't figure out why his baby brother is upset.
Sebastian guesses there was something to all Lucifer's whining about his family not knowing him or understanding him. He also realises that he has an opportunity, here, to maybe help the relationship out by explaning that Lucifer missed his brother.]
Well, in that case, I'm certain you know nothing need happen to him, necessarily, for that streak to emerge.
[On the other hand, two grown "men" can sort out their own relationship drama without Sebastian's help.]