Hm, I don't think that's very likely... Because even though the relationship between Jun's parents failed they were still able to meet again, even if it was only just to fight horribly.
Besides, for Lu Xuan to have it now would have to mean Jinha/Jun's stepmom and Lu Xuan's family somehow knew one another, yet Lu Xuan really didn't seem to know anything about Jun when they first met...
The Envy Stone only breaks people apart; I don't think it can force one half of the fate couple to fall in love with the owner of the envy stone.
My guess is that Lu Xuan's father lusted after one of Lu Xuan's parents, and used the stone to break them up (that's why Lu Xuan said he got it from his father.) But then, he couldn't convince the person he wanted to marry him. Since the envy stone technically did it's job, it probably lost it's power like the fate stone, which is why he passed it on to his son.
Too bad Jun's mother is dead. I would've liked to see Jun and Siwon foo on an adventure to get them back together. ╥﹏╥
Even the raws haven't addressed this yet, but do you suppose the womam Jinha's dad dumped her for had the Envy Stone herself?