No, it is not appropriate for adult siblings to have a romantic or sexual relationship with each other. Such a relationship is considered incestuous and is generally illegal in many jurisdictions due to the potential for severe psychological and physical harm to both parties involved.
Sibling relationships are meant to be based on familial love, trust, and support. Engaging in any form of intimate or sexual relationship between siblings is not only socially unacceptable but also ethically and morally wrong.
If you find yourself in a situation where you or someone you know is experiencing such feelings or contemplating such a relationship, it is essential to seek professional help from a therapist or counselor who can provide guidance and support in dealing with these emotions and navigating the complexities of family relationships. They can offer a safe space to explore these feelings and work towards healthier ways of relating to one another as siblings.