Out of the 13 siblings in their family, Taro and Syn were one of the closest. The Jolteon had taken care of his little brother even since they were just young eevee. Even after Taro evolved into a strong Jolteon, he still spoil his little brother like no tomorrow.
It all changes when Syn evolved into a Sylveon. Syn possessed effeminate body that he almost passed for a female unless they look at his boy parts. And Taro, no matter how a dutiful son he is, succumbed to temptation and raped his little brother out of pure lust.
Their relationship changed afterwards. Syn began to avoid Taro, while Taro did all he could to atone for what he did. He even resigned to be hated by his beloved little brother forever for what he did.
It takes a bit of time, and Taro's confession that Syn is actually very beautiful that mend their relationship somewhat. And the two started to call each other brothers again. With added benefit of sexual time between them, only this time it's consensual.
Their relationship get ups and downs through the time. Eventually the two brothers admitted their love at each other. Which resulted in them both running away from home in fear of the family knowing about their incestuous relationship.
Now they live in secluded home in the outskirt of the forest. Living their lives happily, just both of them.