In all breeds of horses I have been associated with, they have reserved the term '1/2 brother or sister' for foals out of the same mare. Horses sired by the same sire are always referred to as just that -- 'Horses sired by the same sire. '
If you have two foals that are sired by the same sire (or by full brothers) and out of full sisters, they are referred to as "full brothers in blood".
If a person wants to be taken seriously and as being 'knowledgeable', then that person must know how to use the terminology correctly as set up by the industry. The industry uses these terms in this way. This is how horses are listed in catalogs. This is how auctioneers refer to horses. Horses being referred to as 'half brothers' and as 'sired by the same sire' are not ever used interchangeably by knowledgeable horsemen -- like it or not!!!