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.
Matthew 1:24
Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife:
25 And knew her not till she had brought forth her firstborn son: and he called his name JESUS.