I'm not sure of the specific details of Dantdm and Jemma's love story. Maybe they met in a certain place, gradually got to know each other, and found that they had common interests and feelings for each other.
Daniel was a man of strong faith. He was among those taken into the king's service. He had his own beliefs about food and was not willing to consume the king's food which might have been against his principles.
Since I don't know the specific 'big brother daniel story', I can't really say who he is exactly. He could be just an ordinary boy in a family who has the role of a big brother.