May a stepfather or a half-brother act as marriage guardian?

A stepfather cannot be guardian because he is not a guardian by kinship; a half-brother through the mother only is not a guardian either, but a half-brother through the father may be, in the order of guardians.
Guardianship in the Shafi'i school follows the male agnatic relatives on the father's side: father, grandfather, full brother, paternal half-brother, and so on. A maternal half-brother and a stepfather have no agnatic tie, so they are not included. If no guardian by kinship exists or is eligible, a judge-appointed guardian gives the woman in marriage. A stepfather who raised you can be honoured at the event, for example by giving a speech or leading a prayer, without being guardian. If a guardian lives far away, he may appoint the officiant to give you in marriage. See the order of guardians.