Marble is sad, Limestone is angry and Pinkie is happy Maud is neutral on the emotional spectrum so it’s only natural that she can interact with her easier than her other two sisters.

@Frozen Fox  
Oh i though you were agreeing with h-rainbow comment, read them in sequence and though they were related even if there was no reply, didnt even notice the date…lol
 
 
derp

@Kj2  
I was referring to the original image, but you make a good point too.

@Frozen Fox  
@headlessrainbow  
Maybe they are best friends but in a more protective way
 
Maud and Pinkie are direct opposites and complement each other just like Lime and Marble if ya think on it

I thought the same thing…

At first I assumed that since Pinkie and Maud were best friends, Limestone and Marble must also be best friends. I guess not.

Limestone probably wouldn’t ever want to leave. She seems convinced that she’s the only thing keeping the rock farm operating. I’m not sure if that’s accurate or not, but she definitely seems rooted there.
 
Marble’s never really had a reason to leave… until now.

Marble wouldn’t even get on the train.
 
 
Limestone would get kicked off after about 2 minutes.

@Tszeb  
As I’ve been saying for a bit, that’s sort of accidental on her part. She’s basically a bizarre case of a Pony with social magnetism with little social grace.

That awkward situation, when Maud is the most sociable sister after Pinkie.

@Mort  
That feature may _{(or may not)} come soon, until then you can just ask a mod/report your comment.  
Also I fixed that one for you.

She should have no trouble with Fluttergrey.

@ColdhardSilver  
Yes, that. That’s the thing Maud and Pinkie are that Marble and (definitely) Limestone aren’t.
 
Well, Marble has some, but only because of the whole passive shy love toy thing.

@Darth Sonic  
I do phrase you’re looking for is social magnetism. Be it for something good, bad, funny, etc just them doing it causes others to take notice i.e. social magnetism.
 
Pinkie Pie and Maud Pie of course.  

Pinkie’s family is all over the place in terms of emotions. You have the shy one, the sadistic one, the mellow one, the hyper one, and the two uptight parents.

@ColdhardSilver  
Well, yes, but a certain kind of charisma. Just saying charisma would imply a certain social grace neither Pinkie nor Maud possess. They lack social grace, yet possess an undeniable charisma from just being themselves. Couldn’t find a better word then presence, though I guess aura works?
 
As for Force of Nature, that really should be self explanatory. What else do you call a Mare that makes spacetime her bitch in the name of comedic timing, and another who can tear the very land asunder with her bare hooves, other then Forces of Nature?

@ColdhardSilver  
Bah, story of my life. My (nonexistent) kingdom for an edit button.
 
Yeah, Maud = controlled.

@Darth Sonic  
I’m sorry, but I don’t follow. What do you mean by “force of nature” and presence? Or are you just using them as synonyms for charismatic?

@ColdhardSilver  
I never accused Maud of not being socially inept. I’m just saying she hypercompensates for it through sheer force of presence, much like Pinkie does.
 
Marble and Limestone are also socially inept. But unlike their (apparently middle) sisters, they lack the “Force of Nature” presence to compensate. So while Maud and Pinkie are strangely but undeniably charismatic in spite of their social ineptitude. Marble and Limestone are just Socially Inept.
 
There’s a reason Maud has such a massive fanbase.

Limestone a total bitch if you ask me I kind of hate being around people who have such a mean way of talking to others, and her sister took the cake.

@Darth Sonic  
The way I see it they’re all socially inept in some fashion.  
Marble is too shy to speak (she apparently lets Pinkie speak for her) or interact with others.  
Limestone is very aggressive, controlling, and immediately distrusting of others.  
Maud may not intend to put anyone off, but her stilted expressions make her hard to read and her extremely neutral mood ends up killing others’ at first.

@Tragiclady  
It nearly did for Applejack lol

@Masem  
Guys, let’s be honest with ourselves. Despite her intro episodes insistence towards the contrary, Maud is actually a very charismatic mare in a passive sort of way. Not with her words, but with her presence, the feeling that you are witnessing a force of nature.
 
Marble and Limestone on the other hand, while more normal, are also social idiots. One can’t say a word, and the other is too busy chewing heads.

@Mort  
You accidentally typed Marble in place of Maud for the last comparison.

@deathsia  
I kind like how the writers made each of her three sisters a personality contrast to Pinkie without making them clones of each other.
 
Pinkie = friendly <-> Limestone = grumpy  
Pinkie = outgoing <-> Marble = shy  
Pinkie = emotional <-> Maud = controlled
 
I kinda like that symmetry.