Greetings,
Use of the word “firstborn” does not indicate that there was a “secondborn”.
Something from this very site on the existence of Christ’s brothers:
catholic.com/magazine/online-edition/jesus-had-brothers
Please remember that the notion “Mary had sex with Joseph and had other kids” is an innovation. Beyond modern evangelical interpretations, there is no literature extant that supports this view until well after the reformation. Catholics are just trying to stay true to the doctrine of the primitive Church on this matter.
From practical concerns, I would doubt very highly that a woman who birthed God-Incarnate would be interested in rearing other children. They could simply never compete with their elder half-brother. “You always treat Jesus like he’s extra special!”
Mary & Joseph: “Well… He kinda is…”
If a couple wanted to avoid having more kids in that day, they had to refrain from sex. This is often forgotten in the modern evangelical concept of marriage where some form of birth control is an expected matter of course. Remember that prior to the American 1930’s, this was not the case. “Sex” and “kids” were inseparably linked for the overwhelming majority of human (and Christian) history.
TAMUCCStudent and others also made excellent points with scripture references like Song 4:12 and 2 Samuel 6:7.
As a last, the concerns that Mary and Joseph wouldn’t have appeared married to people is a non-issue. I doubt they walked around telling random people “Hey! That kid isn’t Joe’s! He’s never touched me!”. Verily, that man is worthy of his sainthood…