Jim
He has an older brother called Sam. While Sam birth name was George like their dad but since he was born he'd been called Sam.
Spock
Spock was a miracle and as they say miracles only happen once. He did however have an older half-brother but they are not close, he'd been an adult when Spock was born.
Leonard
His parents decided that it would be easier to only have one child due to the fact his father worked a lot.
Nyota
She has an older brother Davi, he a lawyer they talk. She hasn't told her brother she was dating a Vulcan until she had graduated.
Hikaru
He was a lot of siblings, two half brothers Akio and Daiki from his fathers frist wife. Then two older full brothers Ryota and Tadashi junior (TJ). And lastly two little sisters Kira and Malia. He also has some other half-siblings from his father many out of marriage relationships.
Pavel
Pavel is an only child, his parents tried for another baby a few years after he was born but the baby boy was a stillborn.
Scotty
Scotty was an only child he thinks, his dad may have had other children but his doesn't know.
Jaylah
She has two little brothers but she doesn't know that.
Carol
Carol was an only child.
Christine
She was a little sister Rebecca and and older brother John.
Gaila
As she didn't her dad she most likely has siblings.