The situation you described, where siblings who grow up together and are close as adults fall in love, get married, and start a family together, is generally considered taboo and illegal in many societies due to the potential for serious ethical, moral, and legal issues. It is considered incestuous and is prohibited in most jurisdictions due to concerns about genetic risks, potential harm to family dynamics, and the potential for abuse of power dynamics.
Incest laws exist to protect individuals from potential harm and to maintain the integrity of family relationships. Such relationships can lead to significant emotional, psychological, and legal consequences, and they are generally not accepted or supported by society.
It is essential to respect legal and cultural norms and to prioritize healthy, consensual, and appropriate relationships. If individuals find themselves facing feelings that they consider inappropriate within their family, seeking professional counseling and support is crucial to address and navigate these complex emotions in a responsible and ethical manner.