περι του υιου αυτου, του γενομένου εκ σπέρματος Δαυῒδ κατα σάρκα/peri tou hiou autou, tou genomenou ek spermatos David kata sarka/ concerning his son, the one born from the seed of David according to [the] flesh. So he was born in the flesh. And, in fact, born jewish: γενόμενον υπο νόμον/genomenon hypo nomon. And he died. And if Paul thought he never walked on earth, it is especially strange that he ate with his followers: ο Κύριος ᾿Ιησους εν τη νυκτι ᾗ αρεδιδετο ἔλαβεν ἄρτον.../ho kyrios Iesous en te nukti e aredideto elaben arton.
So despite the fact that Paul isn't interested in the earthly Jesus, isn't writing a gospel, and so forth, it is nonetheless quite clear from his letters he believed Jesus walked on earth. And, of course, that he had a brother.