My twin brother...solid Presbyterian...fell in love with and married a gal who was Catholic. Had a Christian wedding done by a local pastor. Later, in order for his (their) two children to be recognised by the Catholic Church as legitimate, he had to go through all of the rituals to fully convert to the Catholic religion and then have a second wedding but it be a Catholic wedding done in a Catholic church. THAT my friends is MY experience as a witness to how the Catholic church works.
Did your brother and his wife marry before Vatican II? (Vatican II made many changes in the mid-1960’s.)
I'm guessing that your brother is a VERY HAPPILY NARRIED CATHOLIC MAN!
Nothing in canon law requires any of that. Nothing to do with legitimacy. Nothing to do with the kids. Period. It's all down in black and white, and when in doubt ask the tribunal.
You've been played . . . by somebody. Not sure who.