Yes, he is Guy's younger brother.
Sebastian
Sebastian is Viola's brother. They strongly resemble each other.
Jamal Thomas
Viola and Sebastian's father was also called Sebastian. See Viola's line in Act 5 "Of Messaline. Sebastian was my father. Such a Sebastian was my brother too."
Yes, Edward is the eldest brother of the trio. The ages from oldest to youngest are as follows: Edward, Dominic and Sebastian. I know he's his brother because my friend knows Sebastian. Hope that's helped!
Johan Sebastian Bach was raised by an older brother, saw it in "who wants to be a millionaire".
He has a younger brother named Sebastian.
He is actually Jonathan, Clary's real brother.
Yes, Clary from the "Mortal Instruments" series by Cassandra Clare has a brother named Jonathan (also known as Sebastian Verlac or Sebastian Morgenstern). He plays a significant role in the series as a complex and dark character.
Sebastian Cabot had two brothers. Their names were Sancio (younger) and Ludovico (older).
There is no written record of any siblings that Sebastian may have had.