There is no real evidence who the disciple was that Jesus’ loved. Conjecture based upon the probability that the author of the Gospel is the disciple leads people to conclude that this is the case.
Joseph Benson writes:
Jesus was now in the depth of his own sufferings, yet when he saw his mother and her companions, their grief greatly affected him, particularly the distress of his mother. Therefore, though he was almost at the point of death, he spake a few words, in which he expressed his most affectionate regard to her. He saith, Woman, behold thy son — Meaning John. His words were intended to assure her that that disciple whom he loved would, for the sake of that love, supply the place of a son to her after he was gone; and therefore he desired her to consider him as such, and expect from him all the duty of a son. And — Besides expressing great filial affection toward his mother, he gave the beloved disciple also a token of his high esteem.He saith to him, Behold thy mother — To whom thou art now to perform the part of a son in my place; thus singling him out as that disciple on whom he could most depend to fulfil that duty, and thereby conferring upon him a peculiar honour. And from that hour — That is, from the time of our Lord’s death; that disciple took her unto his own home — And maintained her; Joseph, her husband, it seems, being dead. Thus, in the midst of the heaviest sufferings that ever human nature sustained, Jesus demonstrated a divine strength of benevolence. Even when his own distress was at the highest pitch, his friends had such a share of his concern, that their happiness for a while interrupted the feelings of his pains, and engrossed his thoughts. 
The non-Reformed (non-Calvinist) Protestant position is that Mary clearly was a sinner (Romans 3:23), who had been saved by faith (Ephesians 2:8). Mary had been born into sin, but because of her faith, even though she was a virgin, she was selected to be the Earthly mother of the offspring that she had conceived through the power of the Holy Spirit. The Catholic Bible clearly states that Mary was in need of a Savior:
And Mary said,“My soul magnifies the Lord, and my spirit rejoices in God my Savior. (Luke 1:46–47)
When the Bible alone becomes the rule of faith, tradition has to be cast aside. The Bible clearly states that Jesus had other brothers and sisters, but there is no evidence of her having given birth to them. The reason for this is the focal point of the Scriptures is what God is doing to bring people to salvation through Lord Jesus Christ.
The passage from John’s Gospel has been used to support a tradition that Mary was always a virgin and Joseph had died by 30 AD; therefore, John was the disciple best suited to take Jesus’ place.
And that is what the soldiers did. Meanwhile, standing near the cross of Jesus were his mother, and his mother’s sister, Mary the wife of Clopas, and Mary Magdalene. When Jesus saw his mother and the disciple whom he loved standing beside her, he said to his mother, ‘Woman, here is your son.’ Then he said to the disciple, ‘Here is your mother.’ And from that hour the disciple took her into his own home. (John 19:25–27)
The fact that the disciple took Mary into his own home presents her not having any other children as the easiest view to understand. For this does not make the same sense if Mary had other children. However, we are not privy to all the facts, so we cannot decisively conclude that this is the case. Scripture does state that Mary did have other children. Jesus is recorded also saying that those who do the will of God possess the same status as His mother.
And his mother and his brothers came; and standing outside they sent to him and called him. And a crowd was sitting about him; and they said to him, “Your mother and your brothers are outside, asking for you.” And he replied, “Who are my mother and my brothers?” And looking around on those who sat about him, he said, “Here are my mother and my brothers! Whoever does the will of God is my brother, and sister, and mother.” (Mark 3:31–35)
The idea that Jesus would suggest that another person whom He loved (whether John or someone else) would be His proxy is not difficult to understand. Mary would have had an attachment to Jesus and an appreciation of Him as His mother that nobody else could. Jesus being able to know the hearts of His disciples and his brothers and sisters would have not difficulty in choosing who would be best suited to comfort Mary.
The Babylonian mother and child idolatry is rejected by the Protestant Church, as is the cult of the Queen of Heaven and Mother of God, which draws on this text in the Gospel of John (19:25–27) for support to the exclusion of the other texts.
In all probability, the reason that this particular anecdote is included in the Gospel of John is so people will understand that the woman who bore the human offspring for the pre-existent Son of God to enter at birth (Hebrews 10:5) is merely a frail human, who had no other children willing to look after her. Moreover, the woman was instructed to consider the disciple as her son and not Lord Jesus Christ. This element of distancing Himself from Mary was pointed to earlier in the Gospel:
And Jesus said to her, “O woman, what have you to do with me? My hour has not yet come.” (John 2:4)
Many, even among Protestants, are somewhat shocked at the suggestion that Jesus rejected Mary as being His mother at this point. But the instance described in chapter nineteen of the Gospel indicates that Jesus could well have been distancing Himself from her and preparing her for the time when He would leave her. In which case, in all probability, although Mary had other sons and daughters, her attachment to Jesus was such that they understandingly had already distanced themselves from her.