Lady Mairi Campbell is the sister to the current 5th Earl of Argyll.  She
did not want to be beholding to her brother for her keep, so she convinced
him that she was worth far more to the Clan Campbell by being in Court and
keeping an eye on Court affairs and reporting back to her brother.  (He, by
the same token, found it easier to keep his sister in Edinburgh, than put up
with her demands at home.)

Make no mistake, Lady Campbell (Daughter of the 4th Earl) is loyal to the
Queen, then her Campbell Clan -- in that order.  She would never betray her
Queen, nor her family.  And in fact, on more than one occasion, has paid a
goodly price to save one or more of her kinsmen. 

A side note:  She is still a devout Catholic, with strong empathy for Queen
Mary.  However, Lady Mairi's family have switched over to the Protestant
Faith, and she was expected to follow suit.  Therefore she has a duplicitous
role to play at Court -- maintaining her secret true beliefs, and outwardly
putting on a show of following the "new" faith of her family.