Saint David of Wales was born to the Welsh royalty, the son of King Sandde, Prince of Powys, and of Saint Non. It is unknown if he had any siblings. He neither married nor fathered any children.
If Lucy had any siblings there is no record of them.
If Matthew had any siblings I can find no reference to them.
If Valentine of Rome had any siblings there is no record of them.
If Monica had any siblings, there is no surviving record of them.
There is no mention in any biographies about any possible siblings of Patrick.
Historical records are unclear about Saint Cecilia's siblings. Some sources suggest she had brothers, while others do not mention any siblings. The focus is often on her life as a martyr and patron saint of musicians.
If Sophia had any siblings, there is no record of them.
If Barnabas had any siblings we have no record of them.
There is no definitive information about Saint Genevieve having any siblings. She is commonly believed to have been an only child.
Saint David of Wales was never married and had no children.
If he had any brothers or sisters, there is no record of them.