If Mary had other children, wouldn’t one or more stand with her in her darkest moment?
Not necessarily. Her children were likely grown up (Jesus was about 38), they and Mary were not an integral part of his peripatetic ministry, and his conjoint siblings were not enthusiastic about His gospel of the kingdom of heaven nor that of the Kingdom of God. Early in His public ministry at His home synagogue, in the presence of his family:
"But Jesus said unto them, A prophet is not without honour, but in his own country, and among his own kin, and in his own house" (Mk. 6:4 AV).
"And Jesus said to them: A prophet is not without honour, but in his own country, and in his own house, and among his own kindred" (Mar 6:4 DRB, Gk inverted here in translation to partially linguistically obscure Jesus' relationship to his brothers and sisters).
But also, this event took place overnight. without opportunity to be there of anyone farther than walking distance. I think only those accompanying Him for Yom Kippur were nearby.
nly John, the beloved, her sister the wife of Cleopas, and Mary of Magdala were with her at the cross. "Then saith he to the disciple, Behold thy mother! And from that hour that disciple took her unto his own home" (Jn. 19:27 AV)
It seems clear that John took her away then, since in the synoptic Gospels the only other women mentioned observing the noonto three events were looking on from afar. Mary apparently was not there for the interment, either.