My guess is that they WEREN’T there, probably having no wish to be associated with Him and risk crucifixion themselves.
And that is probably why Jesus picked John, someone who demonstrated love and faithfulness and courage regardless, willing to risk life and limb to display his loyalty.
Why not pick someone of John’s integrity if that were the case?
When my kids were little and we were thinking of who we would want to raise them should something happen to both of us, there were friends we would easily have picked over some of our own family members. We trusted them better.
Do you have any better answer?
Yes, I have a better answer: The Blessed Virgin Mary never bore any other children than the Christ Child, and that is why only She and John were at the foot of the Cross.
Christ entrusted His Church to St. Peter. Yet Peter denied Christ three times and also was not present at the foot of the Cross.
Why did Christ entrust His Mother to St. John, but His Church to St. Peter? And where were all these so-called "siblings" at the Pentecost or the Ascension?