her > (Self-evidently Britomart's; but if we take this to mean "Malecasta's", lines 7-9 of the stanza contain doubles entendres on "tilt", "free", "seruice", "ought" (in the sense "rendered"), and "hold of her")
4 For they all seemed courteous and gent,
gent > noble
5 And all six brethren, born of one parent, 6 Which had them trained in all civility, 7 And goodly taught to tilt and tournament;
goodly > excellently tilt > {Take part in a tilt, a contest between two mounted knights with lances, who ride on opposite sides of a barrier and score by attaints (touches, strikes, sometimes involving the loser being unhorsed) and by lances broken} tournament > {Take part in a tournament, an exercise in which two groups of mounted knights fight with blunted weapons}
8 Now were they liege men to this lady free,
liege men > vassals, faithful followers (thus pledged at 301.30:5) to > of
9 And her knight's service ought, to hold of her in fee.
ought > owed hold of > be held by fee > absolute possession (a feudal term)
301.45