In Ramayana, Shanta (शांता) was the daughter of Raghuvamshi-Suryavamshi King Dashrath and Kaushalya. Shanta was the real elder sister of Lord Rama, one of the most prominent and prolific incarnations of Lord Vishnu. She was also the half-sister of Lakshamana, Bharata, and Shatrughna.
Unknown to many today, Shanta has not been given much light in the epic Ramayana. This could be partly because no events directly surrounding her in the Ramayana and partly because she was given to King Romapada for adoption at birth by King Dashrath much before the birth of Lord Ram.
Adoption by Romapada
Once King Romapada, who was the King of Anga kingdom and a good friend of King Dashrath, visited the Ayodhya kingdom along with his wife Vershini. Romapada had every comfort and happiness in life except for a child. On sharing this with Dashrath and Kaushalya, Rompada and his wife pleaded and asked them for their daughter Shanta for adoption. Hearing their misery, King Dashrath and Kaushalya decided to give their only child yet to Romapada. Hence, King Romapada and Queen Vershini became the foster parents of Shanta.
King Romapada was also the co-brother of King Dashrath as their wives, Vershini and Kaushalya, were sisters. This way, King Romapada & Queen Vershini were also uncle and aunt of Shanta.
Marriage to Rishyashringa
Shanta was married to legendary ancient Indian sage Rishyashringa (ऋष्यश्रिंगा), son of sage Vibhandak (विभांडक).
For a very long time, the Anga kingdom was suffering from draughts and famines because of no rain in the region. With an earful of tales of Rishyashringa's power and atrocities, King Romapada gave an invitation to the rishi to visit the Anga kingdom and accept their service. Rishi Rishyashringa accepted the invitation and visited the kingdom. As soon as he stepped into the Kingdom, the place was showered with heavy rain. Thereafter, the king personally welcomed him and did the great service of the Rishyashringa. Romapada later asked Rishyashringa to marry his daughter Shanta, hence Rishyashringa became the son-in-law of both Romapada and Dashrath.
According to legends, Rishyashringa also performed the Putra Kameshthi Yagya or Progeny fire ritual for King Dashrath as the king was looking to have a son to continue his dynasty but had no luck in the past several years. Because of the yagya performed by the Rishyashringa, King Dashrath begets 4 sons - Rama, Bharata, and twins Lakshaman and Shatrukhana from his three queens, Kaushalya, Kaikeyi and Sumitra, respectively.
References
https://www.hinduismfacts.org/the-story-of-lord-ramas-sister-shanta/
https://en.wikipedia.org/wiki/Shanta