Same way they reconcile that Mary was sinless and only had one child Jesus.
Oh sheesh...
Mary being sinless--how many other times in the Bible did the angel Gabriel run around telling people, personally, "Hail, full of grace"? And if she's full of grace, how can there be any room for sin? And if she gave birth to Christ the Man, how can something pure come from something dirty? Remember, Christ was true God AND true Man.
One child, Jesus--On the Cross, Jesus told Mary that her son was John, and John that his mother was Mary. Does that make John Jesus' brother? Or perhaps you should look at the usage of language and realize that you cannot apply our translations to their meanings:
"When trying to understand these verses, note that the term "brother" (Greek: adelphos) has a wide meaning in the Bible. It is not restricted to the literal meaning of a full brother or half-brother. The same goes for "sister" (adelphe) and the plural form "brothers" (adelphoi). The Old Testament shows that "brother" had a wide semantic range of meaning and could refer to any male relative from whom you are not descended (male relatives from whom you are descended are known as "fathers") and who are not descended from you (your male descendants, regardless of the number of generations removed, are your "sons"), as well as kinsmen such as cousins, those who are members of the family by marriage or by law rather than by blood, and even friends or mere political allies (2 Sam. 1:26; Amos 1:9).
Lot, for example, is called Abraham’s "brother" (Gen. 14:14), even though, being the son of Haran, Abraham’s brother (Gen. 11:26–28), he was actually Abraham’s nephew. Similarly, Jacob is called the "brother" of his uncle Laban (Gen. 29:15). Kish and Eleazar were the sons of Mahli. Kish had sons of his own, but Eleazar had no sons, only daughters, who married their "brethren," the sons of Kish. These "brethren" were really their cousins (1 Chr. 23:21–22).
The terms "brothers," "brother," and "sister" did not refer only to close relatives. Sometimes they meant kinsmen (Deut. 23:7; Neh. 5:7; Jer. 34:9), as in the reference to the forty-two "brethren" of King Azariah (2 Kgs. 10:13–14)."
"Same way they reconcile that Mary was sinless and only had one child Jesus."
What - by making things up?