There’s no scriptural evidence that the “brothers and sisters” were only half-brothers, but that doesn’t mean that there’s “no need” to make assumptions. From an early stage, there is a consistent and widespread (becoming universal) belief in Mary’s perpetual virginity among people who are intimately familiar with the scriptures, and this persists for nearly fifteen hundred years.
Earlier on you said you were willing to bet that the traditional belief did not predate AD 600. In point of fact, it had been dogmatically defined before that date, and a number of the early fathers wrote about it extensively by the fourth century, and some before. According to the Cathlic Encyclopedia, Iranaeus promoted the doctrine; he died in AD 190 or AD 191.
You dismiss the Protoevangelium as “written at least two centuries after the death of Jesus. I’m no expert, but what I have found through Google suggests that it was written between AD 120 and !D 170, closer to one century after the death of Jesus". I haven’t seen any source date it as late as the third century, as you suggest, though I am happy to be corrected on the point. The Protoevangelium is certainly not canonical; it does not follow that everything in it is false. If you know of modern scholarship which says that it consists of “made-up stories”, now would be a good time to point us to it. More likely it is a compilation of pre-existing stories and beliefs, some of which are taken from the Gospels, some of which are noncanonical but also appear in other sources, and some of which do not appear elsewhere. There is no reason to assume, however, that even those elements which do not appear elsewhere are simply “made up”.
It was cited, not as evidence that its contents are true, but as evidence that the perpetual virginity of Mary was widely believed long before it was dogmatically defined. We know that the Protoevangelium was popular and widely read, if only from the very large number of translations which exist, and from its early liturgical use in some Eastern liturgies.
In other words, this does seem to be an ancient tradition, almost as old as the Gospels themselves. That doesn’t make it true, of course, but it seems to me reasonable to suggest that the age and strenght of this tradition is a factor which it is reasonable to take into account when interpreting the Gospel texts.