Hi MaryJane,
I agree with psalm1's answer and would add more pointers.
1. We know that Mary 'believed' in Jesus in a way which His brothers did not [i]at the time of His crucifixion.
2. This is confirmed by the fact that she was in the upper room praying for 10 days before the Holy Spirit was poured out (and although we don't know that His brothers were [i]not[/i] there, we are sure of their previous unbelief).
3. I believe John the disciple may have been related to Jesus physically, because he knew members of the High Priest's household, and we know that Mary His mother was cousin to Elisabeth, John the Baptist's mother who was of the same priestly descent as her husband.
4. Here is an interesting section in which it appears John has spent the night with kinsmen, and has to choose with whom he is going to stand (them or the other disciples) when there is a sudden moment of decision.
Acts 4
5 And it came to pass, on the next day, that their rulers, elders, and scribes,
6 as well as Annas the high priest, Caiaphas, [u]John[/u], and Alexander, and as many as were of the family of the high priest, were gathered together at Jerusalem.
Why was John [i]there[/i]....? Maybe there are other explanations.
And, I realise that by this time, he has been taking care of Mary since the time of the cross... But, can you imagine what it would have been like [u]for Mary[/u] - aware of these men during her growing years - having to receive their 'hospitality' [i][b]after[/b][/i] they'd had such an instrumental part in her son's death? I'd be sick!
EDIT: Now that I've written all this, I also see it might have been very politically incorrect for Jesus brothers and sisters to have been seen to be siding with anyone - even their mother - who accepted Jesus' ministry and true identity.
Jesus knew His mother would be safe with John and that John had already made the necessary choices about following Him. Perhaps by giving her to John's care, He solved far more many questions that others might have had to face prematurely under that circumstance, than simply about her well-being. Jesus was also looking after the interests of many others.