Can my half-brother through my mother (different father) be my wali?

No; in the Shafi'i madhhab, a brother through the mother only is not a wali nasab, because the relationship runs through the mother. The wali nasab comes only from the father's side: the father, the paternal grandfather, a full or paternal brother, their sons, and paternal uncles. If none of them exists, the right to act as wali passes to the wali hakim. Your maternal half-brother can still help with the ceremony, and can be appointed as a witness if he qualifies. Discuss the order of your wali with the Islamic religious office when you apply for permission to marry. See the order of wali nasab.