Her children are in the wedding party, and I overheard people calling them the groom’s brother and sister.

Just possibly, your son has decided to think of them as his brother and sister, or just finds it less of a mouthful than including the step- prefix. Why are you so angry about him welcoming these children into his family? (especially if they are still actually children by age).

I have four stepsiblings, all acquired in adulthood and after my own marriage, and no we didn't grow up together but they're part of my family now.