When do you use ser vs estar?
It depends on whether you are describing something that can change or something that is unchanging. For example, I am from Alabama is a fact that does not change. In this scenario you would use ser, Yo soy de Alabama. If you are in Alabama however, that is something that can change so you would use the form estar. Estoy en Alabama.
Where do I put the apostrophe?
If you have a house that belongs to more than one person it is the sisters' house (with the apostrophe at the end). If it belongs to one person then the apostrophe goes before the final s. For example, it is my sister's house.
What could the green light in the Great Gatsby symbolize?
The green light is visible but distant from Gatsby which makes it seem like it is related to some sort of goal. Green can symbolize money and perhaps although he is rich he felt the need to get money so Daisy would be with him and perhaps he still connects having more money with a greater likelihood that she will be with him. It could also symbolize envy that he has for Daisy's husband, so the life he leads is distant from Gatsby's own life but he could still see what could have been for himself. It could also be symbolic of the stem and leaves of an actual Daisy showing that he wants to be one with Daisy but by always supporting her and lifting her up.