Many don't realize that Jesus was not an only child when he walked the earth. Although he was the firsborn, he wasn't the only born. He also had at least one cousin as well. Let's start with his earthly parents, Joseph and Mary:

Both Joseph and Mary were descendants from King David (Matthew 1:6-16, 20 and Luke 1:27, 3: 23-32). If you look at all the names in both of these lists, you'll see that they aren't all identical, though both come from the line of David. Both lists mention Joseph, neither mentions Mary. This is because when it came to family lineage, it was considered to be through the father, even if it included the wife's family. At any rate, Jesus's earthly existence was through the line of David in both cases. This is to fulfill prophecy that states Jesus would come from the stump of Jesse (who was David's father), (Isaiah 11:1, 10) and prophecy that specifically names David as an ancestor of Jesus. When Joseph married Mary, she was a virgin, but pregnant with Jesus through God's Holy Spirit (Matthew 1:18-24, Luke 1:26-38). Although married to her, Joseph did not consummate the marriage until after Jesus was born (Matthew 1:25).

According to scripture, Jesus had earthly brothers and sisters as well, therefore Joseph and Mary did eventually consummate their marriage. Christ's brothers' names were James, Joseph/Joses, Simon and Judas (all were common names back then) (Matthew 13:55, Galatians 1:19, Jude 1:1). A reference to sisters is also mentioned (Matthew 13:56, Mark 6:3), and one particular verse seems to be referring to one sister as being named Salome (Mark 15:40).

Just a few months before Jesus was born, his earthly cousin, John the Baptist, was born. This is the bible account of John:
Zechariah was a priest in the nation of Israel, he was married to Elizabeth. Elizabeth was a barren woman and they were both well along in age at this point (Luke 1:5-7). One day, as Zechariah was performing one of his priestly assignments, Yahweh's angel appeared to him. Zechariah was initially frightened by this surprise, so the angel calmed his fears. The angel told him that his wife would bear him a son, who was to be named John (Luke 1:8-13). John was to become a Nazirite (which is not the same as a Nazarene)* (compare Luke 1:15 with Numbers 6:3-4). John was to direct people back to God's kingdom, and he was to be the foretold Elijah to come, (Compare Malachi 4:5-6 with Luke 1:16-17, Matthew 11:13-15, 17:12) and the foretold voice in the wilderness (Compare Isaiah 40:3 with Matthew 3:3, Mark 1:1-3). Zechariah thought this was incredible, and voiced his doubts to the angel. As a result, Zechariah was rendered mute until the actual birth of John occured (Luke 1:18-22). Soon, Elizabeth did indeed become pregnant, and she kept herself out of public for five months (Luke 1:24-25) It was during her sixth month that the angel Gabriel appeared to Mary and spoke to her regarding her human motherhood for Christ (Luke 1:26-38). Here Gabriel refers to Elizabeth as Mary's cousin (Luke 1:36), thereby showing a blood relationship. Mary was thrilled and immediately went to visit Elizabeth (Luke 1: 39-40). When unborn John heard Mary's greeting upon her arrival, he leaped in Elizabeth's womb (Luke 1:41-45). Mary stayed with Elizabeth for about three months before returning home (Luke 1:56). Soon, Elizabeth gave birth to John. The eighth day after birth they went to perform the circumcision ceremony on him and to officially name him. Once he was officially named, Zechariah's powers of speech returned to him (Luke 1:57-64). John continued to grow and become strong (Luke 1:80). Later, in the fifteenth year of Tiberius Caesar's reign and Pontius Pilate was governor of Judea, God's declaration came to John (Luke 2:1-2). As a result, John started to perform baptisms in the Jordan River in symbol of repentance of sins in fulfillment of prophecy (Mark 1:9, Luke 2:3-6).

Other than these references, I haven't found any other evidence of Jesus' human relatives in the bible.

*A Nazirite was a person who was set aside especially for Yahweh's service. It could be a male or female. During the period set aside for Nazirite service, the person must abstain from intoxicating drinks, eating neither grapes nor anything made from grapes, nor cut his/her hair and must not touch any dead bodies including those of family members. (Numbers 6:2-12). A Nazarene, on the other hand, was simply a resident of the city of Nazareth (Matthew 2:23).