Marcus was attending the ceremony for several reasons - one, Scotland was an ally and you attended these events of your allies, two, he was marrying into extended family and this is what you did for family, and third, he would like to think he and Ernest were on friendly terms and this is what you did. Never mind it put him into dress robes - which he loathed. As he sat next to Pansy, he held her hand in support during the ceremony. It was was an intense moment to see someone take on that kind of responsibility.
He was ignoring Morrigan next to him as she was being completely unreasonable. They were attending an event - you wore the colors of your House. And that was green. He was not longer her favorite brother - but he would ride that out till Jon pissed her off and she came back to his camp. He gave it a week.
Towards the end of things he was starting to feel uneasy, and once things got wild he moved, covering Pansy with his own body urging her back and away from her cousin, his eyes flying to find Morrigan. He motioned for Jon to get to his fallen sister and started fighting back, "Pansy, get to the guards, and stay low." His voice was firm and calm - his control over his emotions complete, his sister was down, his fiancee was in danger and his ally was being attacked. This was NOT going to be his day.