[aMorePerfectUnion]

If Mary had other children, then why did Christ create a new son for her on the cross?

Christ instructed John to take care of her as his mother. Apparently, at that moment of His death, no other siblings were in attendance.

In any case, Christ commanded. John obeyed.

[agere_contra]

Apparently, at that moment of His death, no other siblings were in attendance.

No. No other siblings were in existence.

Seriously: Christ on the cross ordained a new son for His mother. He didn't do that just because her other sons were stuck in a traffic jam in Damascus.