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.

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...

Where is the conflict?

Who is the lead character?
What is their goal?
What is standing in their way?

It's good to have intrigue but not confusion. If they are not human they are?

Plus a conversation isn't much of a story. What happens during the conversation? This is conversation fully of subtext about their relationship?

I don't see enough here to know what your story truly is and what it is about.

This next sentence may seems a bit hard.

But your logline should show us how clever your story is, not how clever you are. Your logline is a witty little sentence with a sting in the tale. Shows you can write, but I am blind to the real story.