Yang was happy that her sister found a boyfriend.
Even if he wasn't the best, he was kinda a dork, like Ruby. So hey, two dorks.
Yang supported their relationship at first.
It was after she saw how their relationship worked, or more aptly didn't work, that she realized they shouldn't date.
It started small, little things that could be excused as just inexperience.
It was Ruby ignoring schoolwork to spend time with Jaune.
It was Jaune still spending nights with Pyrrha.
It was both of them being clingy whenever they were in the same room.
But during the third year at Beacon, Yang started having doubts.
Ruby and Jaune didn't change much. They didn't mature at all.
Ruby still ate an unhealthy amount of cookies.
Jaune still ate an unhealthy amount of Chicken Nuggets.
When confronted on their habits, they asked. "Why should we change?" They said. "We're happy with how current things are."
When Blake brought up how couples are supposed to help their romantic partners, they said "We're our own person, if something could be handled alone. it should be."
After that Yang started paying more attention to them.
Yang talked to Ruby about her worries, and she discovered that Ruby trusted Jaune completely, but both of them kept secrets.
The second worst moment for Yang, was finding out that they had an open relationship.
That they weren't committed.
The first worst moment, for different reasons, was walking in on Ruby and Jaune having sex. With half of team CVFY. There was whips, collars, costumes and a lot of other toys.
They liked to experiment.
Yang didn't know if she was being paranoid at this point, because now everything they did, she somehow saw the bad parts.
They were competitive. Always trying to one-up another. There would be bands, fireworks, or an excessive amount of glitter.
They always had a better, more extravagant way of saying 'I Love You!'.
When they did argue, it always got heated. A couple of times it ended in a fight.
The first time that happened, Yang nearly killed Jaune for that, until the why and how got revealed.
Ruby wanted to go on a literal suicide mission.
Reports of a massive hoard of Grimm were spotted heading to a village. A village that despised Vale. A village that wouldn't accept help. No mission was made to save the town.
When Ruby found out, it was too late. The village was destroyed. She still wanted to go. To find survivors. The Council was using that town to funnel the Grimm, so it was off limits.
Jaune had to hold her back, she tried breaking free, in the end she took her sorrow and rage out on Jaune.
Afterwards things progressed normally for them.
Like every fight before and after, they didn't change.
Sometimes Yang just watched as they went on wacky adventures.
There was a time Jaune was framed for a crime, a time where Ruby attacked Atlas and for a time was labeled as a terrorist, a time where they accidentally discovered a Dust that can swap Souls, they didn't complain much as it gave them new things to experiment with.
There was much more examples, more moments, more reasons they shouldn't date.
It was okay for now, but what about later? What about their future? About kids? About any responsibilities?
Presently, Yang was leaning against a wall next to a punch bowl, at a New Years Eve party.
She watches as Ruby and Jaune drunkenly and openly express their love by having a heated make out session.
They're happy. Why couldn't she just be happy for them?
She sighs.
Her sigh was echoed by another blonde sister watching the couple.
That was how Yang Xiao Long met Saphron Arc, and started her attempts to break up Ruby and Jaune.