Macha is involved in the quest Enemies Among Us.
She is very worried about her templar brother, Keran, because he used to write to her every day when suddenly the letters stopped arriving. She asks Hawke to speak with the templar recruits in the Gallows to find out what happened to him.
After Hawke rescues Keran, Macha will be with him at the Gallows as Cullen is assessing whether or not he should remain in the order. Hawke can either convince the Knight-Captain to allow Keran to continue his training, or be expelled from the order. Either way Macha will send Hawke a letter expressing her thanks.
If Keran stays with the templars Macha will express concern over what the templars will do to him but recognizes that staying is his choice.