And therefore I look upon everything as a brotherhood and a sisterhood,
Because of what she just told us (that she wants to die full of curiosity), our speaker tells us that she views everything as a part of a brother/sisterhood, part of a community.
Perhaps part of the reasoning behind this sense of community is also that everything dies (death is kind of a great equalizer, puts us all in the same boat).
We also can't help but notice the line break here. "Therefore I look upon everything/" We've already picked up, based on the specificity of her descriptions, that she seems to notice a lot. And this line break suggests that part of the reason she observes things is because of her knowledge of death, and her desire to be open, to learn what there is to learn from what is around her.
and I look upon time as no more than an idea, and I consider eternity as another possibility,
Also because of what our speaker has been telling us, she questions our notions of time, and thinks eternity is also possible.
Since she doesn't seem to reference any heaven (and the cottage of darkness seems to be a declaration that she does not know what happens), it makes this line more mysterious than it would be if she just said "I believe we spend eternity in heaven when we die."
What, then, does she mean by eternity? Is it different from time, because time suggests that everything must change or end?
Eternity seems to touch on some of the same ideas as those suggested by the cottage of darkness – like that existence might continue in (or through, or beyond) death.
Time is obviously related to death and mortality. It's only a matter of time, right? But if we consider that time doesn't exist, or not as we think of it, that makes death suddenly very different. Maybe it wouldn't be something to fear.