Beyoncé’s son born in 2017 is named Sir. Beyoncé and Jay-Z welcomed twins, Rumi and Sir, on June 13, 2017, and announced their names publicly about a month later.
Sir is the younger brother of Blue Ivy Carter, who was born in 2012. The family’s children have appeared in selected musical projects and public events, but their parents generally maintain strong privacy around their daily lives.
Rumi is Sir’s twin sister, making her name the most likely alternative in a question about the twins. Blue is the name of the couple’s first child, not either twin.
The name “Sir” is a title in ordinary English usage, but here it is the child’s given name. Public biographies typically list the children as Blue Ivy, Rumi, and Sir Carter.