The logline lacks a plot.

And if the sisters aren't human? then what are they?

Sounds like a short movie, or a theater piece... Use the plural in sisters... How many sisters?
Is this a post-human species or just a group of aliens? Are they organic beings?
What is their interest in humans? Are they scientists or is this just a casual conversation?
What kind of society are we talking about?
Do they have contrasting opinions about humans? You need conflict...
If it is a comedy you should present something that indicates the genre in your logline.

A group of sisters have a lengthy conversations as they attempt to learn the social, cultural and physiological processes of a mostly unknown and extinct species: Human.

Here's my take, but I'm a newbie...