There are many things which we don’t know for certain but It is probable, based on Eusebius, that the Mary mentioned in as the sister of Mary the Mother of Jesus, is actually her Co-sisters in law (One’s husband’s brother’s wife; or more generally one’s spouse’s sister-in-law, the sister of one spouse in relation to the siblings of the other spouse). first allow me to quote Scripture:
“Near the cross of Jesus stood his mother, his mother’s sister, Mary the wife of Clopas, and Mary Magdalene.” John 19:25
There are 3 Mary’s above. The Blue one is the Lord’s Mother (Mary). The red one is called her sister and is a wife of Clopas. And Mary Magdelene in Green.
Hegessipus tells us:
“And after
James the Just had suffered martyrdom, as had the Lord also and on the same account, again
Symeon the son of Clopas, descended from the** Lord’s uncle**, is made bishop, his election being promoted by all as being a kinsman of the Lord.” Hegesippus. -(170 A.D.)
Eusebius tells us:
“After the martyrdom of James and the conquest of Jerusalem which immediately followed, it is said that those of the apostles and disciples of the Lord that were still living came together from all directions with those that were related to the Lord according to the flesh (for the majority of them also were still alive) to take counsel as to who was worthy to succeed James.They all with one consent pronounced
Symeon, the son of Clopas, of whom the Gospel also makes mention; to be worthy of the episcopal throne of that parish. He was
a cousin, as they say, of the Saviour.
For Hegesippus records that Clopas was a brother of Joseph.”
(Church History 3:11:1-2 [A.D. 312]).
-Eusebius of Caesarea
source:
newadvent.org/fathers/250103.htm
If Eusebius is accurate (and lets consider the fact that he is one of the best church historians), then let me show the following image of how these 2 Mary’s relate:
The above would be the relation suggested of the two Mary’s.
Its just one of many possibilities. Don’t take it as an established fact.