Whenever Luke decides to have a pity party for himself, he calls Mark "the lucky second" (1.19). Despite being two years older than Luke, Mark is the brother who has the most interaction with Luke. Granted, sometimes that interaction is teasing him or calling out "Population Police" while banging on Luke's door. But, hey, that's what you get from the son "who normally had a smart-alecky comeback for everything" (6.11).
Brotherly teaching aside, there is no question that Mark loves his little brother. Mark goes out of his way to remind Luke that's he's safe and that everyone in the family will continue to keep him a secret. And when Luke eventually leaves home, Mark has a serious look on his face, which Luke notes is "strange enough for him" (30.5).