Mark didn't write that Joseph, Simon, James, and Judas (Judas/Thaddeus) are Jesus's siblings, but rather His
"ἀδελφοί" (sing. adelphos; pl. ἀδελφοὶ adelphoi) and
"αδελφαι" (sing. adelphē; pl. αδελφαι adelphai), translated to
"brothers" in English, and it has multiple definitions, e.g., "fellow-countryman", "disciple/follower", "one of the same faith", and "a near kinsman, or relative", e.g., sibling, cousin, nephew, niece, uncle, aunt, etc. As of right now, you've only
assumed the type of kinship that applies is siblings, and an assumption doesn't "leave little doubt" that they were Jesus's siblings.