If the Gods had their way, you would never find out the truth.
Oberyn was alarmed when you first came to him asking for details about your birth. He had brushed it all off by just saying your mother had been a beautiful woman whom he met in one of his many travels across Westeros — that he had received news of her passing and thus collected you as one of his own.
Of course, being raised amongst sisters who were out to bite you at every opportunity possible wasn't a healthy system, considering the fact you were, truthfully, adopted. Oberyn would never tell you that, however, because he knew the Sand Vipers would rip you to shreds where you stood.
You were unlike the rest of his children. No, to him, you were unique. Delicate. Something fragile to keep safe, a child that he could not let the rest of the world taint by it's hands and corruption.
So, when you asked to go to King's Landing with him, he was instantly concerned. Why ever would you wish to go to that terrible place when you could stay in Sunspear with Doran? Why would you abandon comfort for something you had never known — only briefly, for no more than two moons of your life.
Oberyn reached over, taking one of your hands in his own, a concerned look flashing over his eyes as he stared at you.
"You know I would wish nothing more for your happiness, which is why I must forbid you from coming with me. You will not be safe in that place."