Neither disputes the fact that Elijah met her first, and neither dispute which brother is clearly favored. Elena pulling out the dagger—twice—proves her loyalties. But Klaus's persistence pays off in the end, and after a couple centuries she gives him what he wants.
Somehow the three find themselves living together, both Klaus and Elijah with separate bedrooms in opposite wings of the mansion Elena chose. Elena, on the other hand, gets whichever bedroom she chooses to grace with her presence, but she is never alone.
None of them ever bring it up, but they all are perfectly aware of how much she enjoys it; and for the majority of the time, Elena will even admit to being happy.
But then there are those moments where she wishes for more.
"Is it wrong to want more than this?" she asks Elijah one day, after Klaus leaves her in his brother's care for an undetermined amount of time.
"There's always the Salvatore brothers," he says.
Elena sighs. "They wouldn't stand a chance against Klaus, and we both know he's too possessive to let me go back."
"He allows us," is Elijah's simple answer.
"He has no choice," Elena reminds him. Klaus may be a lot of things, but Elijah was the one to first stake some sort of claim to her, which left Klaus little choice but to accept Elijah. In fact, Elena made it a requirement to her deal with Klaus.
"Do you ever think about leaving?" Elijah asks.
Elena's not surprised. After hundreds of years, she has grown accustomed to being compared to her predecessor. "Do you suspect him of trying to kill me again?" she counters.
"He considers you too much of a prize to think about tainting, Elena."
Elena nods, not expecting anything else. "And I wouldn't know where to go," she answers. She doesn't say the 'or who to leave with' but both know it is heavily implied.
It was always part of Elena's Petrova legacy to not be able to choose.