If Jesus Christ had brothers and sisters AT ALL, He would have SINNED in giving care of HIS mother to John.
It is amazing that you would ascribe sin to Jesus Christ rather than believe the full proof of Scripture that Mary had other children, and that she had sex with Joseph like any good wife should.
Holy Scripture is clear that Jesus had siblings, brothers and sisters. It in no way at all says that it was a sin for Jesus to give care of Mary to John.
You people have got to face real life here and smell the coffee.
You would actually rather believe that Jesus sinned rather than believe God's Holy Word.
What a shame.
BTW, if God had wanted to use the word nephew, he would of, in describing Jesus' brothers, because it is used in Scripture in other places. Also the word "cousin", if that had indeed been the case.
Jesus had siblings, Mary was a good wife who had maritial relations with her husband.
Matthew 1:25 And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
References to how Joseph kept Mary virgin prior to the birht of Jesus say nothing to whether they had marital relations after. References to "adelphoi", "brothers" of Jesus are collective references to kinsfolk, as is the common usage of "adelphos" or its Hebrew equivalent, which I forget at the moment, throughout Scripture. For example, Lot is described as Abraham's "brother" even though his genealogy is explicitly given and he is Abraham's nephew.
The scripture is silent on the question whether Mary remained virgin; the tradition is of a single mind that she did, and there is some indirect scripture that corroborates that.
Please refrain, in the future, from reading the scripture in an ill-educated way. Private interpretation of the scripture is forbidden by it. If you are in doubt how to understand a verse from the scripture ask a Catholic or an Orthodox, who can give you an apostolically correct interpretation. I, personally, will be happy to guide you any time you care to ask, as I have done so before.
No Jesus did not sin by asking John to take care of His mother. No one suggests that He did.