Cersei was born first but literally only just. It is said Jamie followed her clutching onto her foot.
"Both." She did not flinch from the truth. "Since we were children together. And why not? The Targaryens wed brother to sister for three hundred years, to keep the bloodlines pure. And Jaime and I are more than brother and sister. We are one person in two bodies. We shared a womb together. He came into this world holding my foot, our old maester said. When he is in me, I feel... whole." The ghost of a smile flitted over her lips.
A Game of Thrones, "Eddard XII"
If the above isn't enough of a hint when you know that in most births the head comes out first it is actually made more explicit later on.
Never, Tyrion wanted to say, but the word caught in his throat. Cersei always resented being excluded from power on account of her sex. If Dornish law applied in the west, she would be the heir to Casterly Rock in her own right. She and Jaime were twins, but Cersei had come first into the world, and that was all it took. By championing Myrcella's cause she would be championing her own. "I do not know how my sister would choose, between Tommen and Myrcella," he admitted. "It makes no matter. My father will never give her that choice."
A Storm of Swords, "Tyrion IX"