"He and she are siblings."
Translation:Él y ella son hermanos.
Pandora is partially right. You use E to avoid cacophony or the repetition of sounds: "hijo y hija" doesn´t sound well and you can´t differentiate the "y", so it sounds like "son -, daughter", instead of "son and daughter". As a rule you use "E" when the next word starts with an I. For example: "Es un hombre poderoso E invencible" (He is a powerful and invincible man). "Es honesto E incorruptible" (He is honest and incorruptible). And finally, remeber that, in some cases the H is mute, like in Hijo/hija, so the first sound is that of an I and thats the reason you sholud use E instead of Y. In the sentence used in this example: Él y ella son hermanos you don´t use E for the same reason: "El E Ella" doesn´t sounds right, you are repeating the E so, you use Y.