Did king Arthur have anybrothers or sisters?

He had no brothers, but depending on the version of King Arthur you read; Arthur may have had one or two sisters.

Morgause (wife of King Lot of Orkney) who was either his sister or half sister.

She was mother of Gawain and Mordred.

Morgan, or Morgana (wife of King Urien of Gore) his half sister.

She was mother of Ywain

In some versions of the story Morgan and Morgause are two different people,

but in some versions the two are combined into one character.