sire > father
3 Who, dying whilom, did divide this fort
whilom > at some past time; a while ago (the intended pointing might also be "Who, dying, whylome did diuide …") divide > {Separate into parts and distribute}
4 To them by equal shares in equal fee:
fee > {Estate of inheritance held in land}
5 But strifeful mind and diverse quality
quality > disposition, character
6 Drew them in parts, and each made others' foe:
in parts > into factions ("part" is cognate with "party", = prin- cipal in legal proceeding) others' > [the others'; or: the other's, in view of lines 7-9]
7 Still they did strive, and daily disagree;