[MHGinTN]

Look at you last sentence. You have answered your own question. Peter was closer to Jesus than even His brothers and sisters during His ministry (see Mark 3:20 and following). And yet Peter denied the Lord under pressure. The supposed brothers and sisters would have had tremendous pressures on them at the end when both Rome and the Sanhedrin were enraged enough to execute Jesus. Who had the courage to be there at the Cross? John. And The Mother of Jesus was likely not as close, physically, as John. Mary might very well not even heard clearly what Jesus was saying.

One last possible point to ponder: Jesus was so fond of John and John's devotion that what Jesus did would have been such a tremendous blessing placed upon John: if there were no step siblings, the blessing is full; if there were step siblings who did not even come to His last moments alive, tragically confirming in their reasoning their worst fears about His preaching against Rome and the Jewish power structure, the fact that John was there and perhaps the youngest of His disciples, being given such a weighty responsibility would have been an enormous affirmation of Jesus's love and trust in John.

[Elsie]

Peter was closer to Jesus than even His brothers and sisters during His ministry

Even with...

Get thee behind me Satan

and

I never knew the man!

[Elsie]

Where was Mary living before being 'given' to John; for the Scripture says...

Luke 9:58
"The foxes have holes," said Jesus, "and the birds of the air have nests; but the Son of Man has nowhere to lay His head."