Is it wrong to have characters intermarrying in my book?

In my book some of their kind intermarry to kept their bloodline "pure". Two of my main teenage characters that are brother and sister have a romantic relationship, but it doesn`t go beyond hand holding and kissing on the cheek except when the council ( she is an important part of it ) is under attack and they think it is the last time they will see each other. Then it describes a full on lip lock. It tells breifly about a few other relationships like that, and tells about how the whole sibling couple thing began.