What is the ruling on kissing and hugging between a brother and his sister, especially kissing on the mouth, and if it is mixed with lust from one or both of them?

Scholars differ regarding the permissible extent of looking at mahram relatives. The more authoritative opinion (ar-rajih) is that it is permissible to look at what is usually uncovered, such as the neck, head, hands, and feet. It is not permissible for a mahram to touch what he is not permitted to look at. Therefore, it is not permissible for a brother to embrace his sister. As for kissing, it is permissible in specific situations, such as returning from a journey, provided it is on the forehead or head and not on the mouth. All of this is forbidden if it is done with desire or if there is a fear of fitna (temptation), as merely looking with desire is prohibited. The brother and sister must be advised to avoid these matters and adhere to the limits of Islamic law.