No, a real sister and brother cannot be friends with "extra benefits." Such a relationship would be considered incestuous and is both morally and legally unacceptable in most societies. Incest involves engaging in sexual or romantic relationships with close family members, such as siblings, parents, or other relatives.
In nearly all cultures and legal systems, incest is prohibited due to the potential for harm, both physically and emotionally, to the individuals involved and any potential offspring. It can lead to severe emotional and psychological consequences for both the siblings involved and their family members.
Healthy sibling relationships should be based on love, support, and care without any romantic or sexual elements. If you or someone you know is facing such a situation, it is essential to seek professional help and guidance to address and resolve the issue appropriately.