Balarama, the Avatar of Vishnu and Brother of Krishna was born not to Devaki,Mother of Krishna but to Vasudeva’s another wife Rohini.
Devaki, mother of Krishna was imprisoned by her brother Kamsa who vowed to kill her offspring fearing his death at the hands of her eighth child.

Bala Rama Devanagari बलराम Sanskrit Transliteration Balarāma Affiliation Avatari of Shesha or Vishnu Weapon Plough, mace Consort Revati.Image Credit.wiki
Vishnu then impregnated the belly of the goddess Devaki with two strands of hair, one black, one white.
To ensure their safety, their essence was transferred before birth to Rohini, who also desired a child.
At birth, Krishna had a darker complexion, while Balarama was born fair.
The other name of Balarama is also Sankarshana, meaning a spirit transferred between two wombs.
‘Devakya Jathare Garbham Shesakhyam
Dham Mamakam,
Tata Sanni Krushya Rohinya Udare
Sanniveshaya’
Bhagavat [10-3-(8-10)]
Shesha which is the abode of Lord
Krishna (Vishnu), took birth as a human in the
form of foetus in the uterus of Devaki.
Goddess Yogamaya had extracted the foetus of Balarama and placed it safely inside the uterus of Rohini,
the second wife of Basudeva. So after birth, his name was Sankarsana.
One day, Nanda Maharaja requested the presence of Gargamuni, his priest, to name the newborn Krishnaand Balarama.
When the Gargamuni arrived, Nanda Maharaja, received him well and requested the naming ceremony.
Gargamuni then reminded Nanda Maharaja that Kaṁsa was looking for the son of Devaki, and if he performed the ceremony in opulence, it would come to his attention.
Nanda Maharaja therefore askedGargamuni to perform the ceremony in secret, and Gargamuni did so:
Because Balarama, the son of Rohini, increases the transcendental bliss of others, His name is Rama, and because of His extraordinary strength, He is called Baladeva. He attracts the Yadus to follow His instructions, and therefore His name is Sankarshana.
—Bhagavata Purana, 10.8.12
In the Bhagavata Purana, it is described that after Balarama took part in the battle causing the destruction of the remainder of the Yadu dynasty, and
witnessing the disappearance of Krishna, he sat down in a meditative state and departed from this world.
Some scriptures describe a great white snake that left the mouth of Balarama, in reference to his identity as Ananta-Sesha.
The place where he departed is situated near Somnath Temple in Gujarat.
Balarama is Shiva.
The Saura Puran states that –
Matsyah kurmo varaha schah
Narasingho atha vamanah
Ramo Ramascha Krishnascha Buddhah Kalki
Cha Te Dasha 11. 15/25 (Soura Purana)
Matsya (fish), Kurma (Turtle), Baraha (Boar),
Narasingha (man-lion), Vaman (Dwarf-man),
Rama, Balaram, Krishna, Buddha and Kalki are
ten incarnation of Vishnu. Here Balaram is
regarded as Vishnu. Now he is regarded as Debata
or God. So Balarama became (Bala+Deva)
Baladeva. Krishna and Balarama are regarded
as Hari and Hara. Here Balarama is regarded as
Lord Siva. Siva is helping Vishnu in every
incarnation like Rama-Laxman in Tretaya Yuga.
In Dwapar Yuga as Krishna-Balarama and in Kali
Yuga they are Jagannath and Balabhadra.
Balarama like Laxamana is a yogi, traveller, silent
worker, renunsation, truthful and Sanyasi as
described in different Puranas.
Citations.
http://en.wikipedia.org/wiki/Balarama
http://orissa.gov.in/e-magazine/Orissareview/2010/July/engpdf/87-90.pdf


