Is it permissible for a brother from the father to see the ‘awrah of his half-sister, kiss her, embrace her, and for her to uncover her head in front of him?

The fiancé's brother is a mahram to her [the fiancée] through her father [i.e., by marriage to her father's son]. Therefore, it is permissible for her to interact with him as she would with other mahrams regarding the wearing of hijab and being alone with him, provided there is no suspicion. This is based on the Almighty's saying: {There is no blame upon them concerning their fathers, or their sons, or their brothers, or their brothers' sons, or their sisters' sons, or their women} [Al-Ahzab:55].