The simple answer is YES! After Joseph married Mary (the birth mother of Jesus) he had merital relations with her. So yes, Jesus did have younger siblings.
- (Matthew 12:47 So someone said to him: “Look! Your mother and your brothers and sisters are standing outside, seeking to speak to you.”)
Although we do not know exactly who many brothers and sister Jesus had, we know that Jesus was not the only child of Joseph and Mary.
Early in Jesus live, his younger siblings did not believe he was the messiah, but after his death and resurrection many of them did put faith in him after all.