User blog comment:Gundam Legilis/A Minor Hiccup In Depth - The plot/@comment-11188061-20140810123449

A legitimate explanation is that Weiss is Ruby's first partner and friend at Beacon. She came to Signal with no one but her sister to accompany her, and as we know she's a loner who thinks her weapon is enough to last her as a friend. Therefore she appreciates Weiss very much, especially since their big argument where both sides learned to fix their attitudes and tolerate each other better.

Weiss also has that childish, innocent side of hers that makes Ruby view her as something as close as a 'same age friend'. Since both sides work towards understanding each other better, it's only natural that Weiss has become Ruby's best friend. She's innocent and direct and young, so when she's attached to a person she just shows her feelings without restraint.

Yang's her sister, so it's, well, not the same. The blonde has also intentionally isolate herself from Ruby so as she could learn to be independent, as seen in Volume 1. But after Ruby is made leader and she found Weiss, Blake and Team JNPR as her new friends, there isn't as much reason to do so anymore.

Still, sibling relationship is just, well, you're closer than anyone, but you just don't walk that kind of close, get what I mean?

Blake has always been quiet, moody and anti-social, and her mood swings comes and goes very suddenly. She also has that mature aura around her that makes people feel that she's older than her age, and that makes it somewhat awkward for Ruby to approach and befriend. How to put it? Ruby likely felt as if she shouldn't joke around and restrain herself more around Blake. She probably sees Blake's death glare often enough she doesn't dare to fool around her that much too.

With Weiss though, she can tease the girl and make fun of her more freely, because although Weiss will show her displeasure and badmouth her in return, she doesn't explicitly stop Ruby from doing it. That's one good thing about Weiss: she's direct and she's forthright - if she says 'Stop that' then you better stop, but if she simply insults or brush it off it really means exactly that. It makes interaction much less stressful since you don't need to beat around the bush and guess a poker face.

At the end of the day, Weiss is just simpler to read and understand. To put it even more bluntly, she's the closest and easiest 'food' out of anyone else LOL.