A group of sister have a lengthy conversation trying to fill in the social, cultural and physiological blanks about a long extinct species: Humans.

The logline lacks a plot.

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

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.