No, it is not appropriate for adult siblings to engage in a romantic or sexual relationship with each other. This type of relationship is considered incestuous and is illegal in many jurisdictions due to the potential for harm and the violation of social norms and moral boundaries.
In most societies, there are strong taboos against sibling romantic or sexual relationships because of the potential for exploitation, manipulation, and harm, both physically and emotionally. Engaging in such a relationship can cause severe damage to the individuals involved and to the family dynamics as a whole.
Healthy sibling relationships are built on love, trust, and support, but they should remain platonic and within the bounds of societal norms. If you or someone you know is facing challenges related to inappropriate relationships, it's essential to seek professional help from a therapist or counselor to address and navigate these issues in a safe and constructive manner.