Mark is studying to be a lawyer, like his father. Clare and Mark have never had a very friendly sibling relationship. Six-year-old Clare relates,
"When I was coming out of the mud room Mark saw me and he said What are you doing asshole? And I said, Nothing, asshole and he pulled my hair and I stepped on his foot really hard and then he started to cry and went to tell." (1.3.80)
When Henry meets Mark in the present, he realizes that Mark recognizes him from somewhere and only much later realizes Mark who shot him on one of Henry's time travels.
Otherwise, although Mark wants to do the right thing by Sharon and marry her because she's pregnant with his baby, he doesn't love her and she doesn't love him. In fact, she doesn't even seem to like him very much. She mentions to Henry once that he got the only nice one in the family.