The Elder brother is explaining his sister's, the Lady's, virtues of chastity and virgin purity. While both brothers worry about their unguarded sister, the elder brother has faith and hope that she will be okay:
Yet where an equall poise of hope and fear
Does arbitrate th' event, my nature is
That I encline to hope, rather then fear,
The elder brother goes on to say that their sister will not be hurt by any evil in the woods. He says that no goblins or ghosts will affect her. He then uses examples from ancient Greece to make his point. He compares the Lady to Diana, goddess of the hunt and of chastity. Diana tamed lions and panthers and was the queen of the woods. The brother also mentions Minerva (Athena) who had a shield with the head of a gorgon on it. (Medusa was a gorgon, a female creature with hair made of snakes who could turn men to stone if they looked at her.) The elder brother's hope is that his sister, the Lady, will be like Diana and Minerva: chaste and wise warriors. He hopes that she will be able to "shield" herself from any evil.