Given the provided information I'm assuming the narrator find Mangan's sister as a goal or an idol of sorts who they see hope wisdom and in this case, light. Light can be used as Mangan's sister's symbolism of great, or ideal.
When the narrator see's the sister he/she/they is reminded of light and goodness.
Fifty cents is a plural noun since it is talking about more than one cent. In the example, you would say, "fifty cents <em>is </em>how much I owe you".
