Sorry; but your claim is a wee bit; shall we say; one-sided here.
I don`t know and would need proof to believe it and do not see any proof as Jesus is the only child mentioned as belonging to Mary.
If Mary had other children why is it not mentioned, why would they be referred to as Jesus`s brothers and sisters if they were Mary`s children?
Your tagine says it all.
I finally stopped laughing ... I really had a hard time believing you actually asked this: “why would they be referred to as Jesus`s brothers and sisters if they were Mary`s children?” FRiend, JESUS is the center of attention for the New Testament, not Mary or the brothers and sisters of JESUS. Even when Mary and the brothers of JESUS show up where HE is teaching and ask for HIM to come have a word with them, JESUS is the center of the scene, but the brothers are mentioned as seeking an audience with HIM.