Lord Rama had a sister, Shantha.

*She was the daughter of Dasaratha and Kausalya.

Later she was adopted by Romapada and Vershini.

Vershini was the elder sister of Kausalya.

Romapada was a friend of Dasaratha and studied with Dasaratha in the Ashram of Vasistha

She was married to Rishi Rishyashringa , son of Rishi Vibhantaka.

Shantha was well versed in the Vedas.

She was the Princess of Anga Desa.

S

“

A king named Dasharatha will be born into Ikshwaku dynasty who will be very virtuous, resplendent and truthful one to his vow.” Said Sanat Kumara, the Sage.”King Dasharatha will befriend the king of Anga and the king of Anga will beget a fortunate girl named Shanta.

Shanta is said to be the daughter of Dasharatha and given to Romapada in adoption, and Rishyasringa marries her alone. This is what Sumantra says to Dasharatha at 1-9-19.

The son of the king of Anga, the earlier king of Anga kingdom, will be known as Romapada, or also know as Chitraratha, and the highly renowned king Dasharatha approaches Romapada. Then king Dasharatha says to king of Anga “oh, righteous one, I am childless and hence I intend to perform a Vedic ritual. Let the husband of your daughter Shanta, Sage Rishyasringa, preside over that Vedic ritual at you behest, for the sake of progeny in my dynasty.

Valmiki Ramaana , Bala Kanda Sarga 11

a~Nga raajena sakhyam ca tasya raaj~no bhaviSyati |

kanyaa ca asya mahaabhaagaa shaa.ntaa naama bhaviSyati || 1-11-3

3. tasya raaj~naH= to that, king [to Dasharatha]; anga raajena= with Anga, king of; sakhyam bhaviSyati= friendship, will happen; asya= his [for king of Anga]; mahaa bhaagaa= fortunate woman; Shanta; naama kanyaa bhaviSyati= named, daughter, will be there.

“King Dasharatha will befriend the king of Anga and the king of Anga will beget a fortunate girl named Shanta. [1-11-3]

anapatyo.asmi dharmaatman shaa.ntaa bhartaa mama kratum |

aahareta tvayaa aaj~naptaH sa.ntaanaartham kulasya ca || 1-11-5

5. dharmaatman= oh virtuous-soul; anapatyaH asmi= childless, I am; shantaa bhartaa tvaya aaj~nptaH= Shanta’s, husband, by you, instructed; mama kulasya santaana artham= for my, dynasty, for progeny, for the sake of; kratum aahareta= Vedic ritual, will preside over.

Then king Dasharatha says to king of Anga “oh, righteous one, I am childless and hence I intend to perform a Vedic ritual. Let the husband of your daughter Shanta, Sage Rishyasringa, preside over that Vedic ritual at you behest, for the sake of progeny in my dynasty. [1-11-5]

* There no evidence that Shanta was Dasaratha’s Daughter,according to Valmiki.

However, Sri.B.G.Kanuja,in his book Immortal Love of Ramayana, writes that Shanta was Dasaratha’s Daughter and was given in adoption to Romapada.

She was born on 12th March,4449 BC.

He quotes Valmiki Ramayana ,Canto 9 (?),Sloka 12?

I am unable to find the reference.

Scholars may clarify.

https://ramanan50.wordpress.com/2017/12/14/ramas-sister-shanta-born-4449-bcabandoned-by-dasaratha/