Hi

I heard from others that our Lord RAM has one sister. Is it true? Any one has idea about this?

Yes, that is true. Rama had an elder sister named Shanta. Dashrath was her biological father but Dashrath gave her to his friend Romapad. Thus Romapad brought her up. Shanta's husband was sage Rishyashring. Rishyashring was the chief priest, who conducted the yajna after which Ram, Bharat, Laxman and Shatrughan were born.

If king Dasarath had a daughter, why she never came into highlight throughout Ramayana?

She is mentioned in Baal-Kand of Valmiki Ramayan. Ramayan contains more details of her husband (Rishyashring) than her. Rishyashring was the one who did the yajna after which the four princes were born.

I read valmiki ramayan including yoga-vasista. I did come across santha daughter of roma-pada whom he gives to Rishyasringa (son of Vibhandaka).

There is a detailed description of Rishyasringabeing lured to roma-pada's kingdom to bring the lost rains. It is here that roma-pada marries his daughter santha to Rishyasringa.

There is no mention of santha being related to dasaratha in anyway. Infact valmiki maharshi even calls rama as dasaratha's "dharma-santan" and calls him his first child. Dasaratha has children only and only after the putra-kamesti yaga where lord vishnu himself promises to be born as four "boys"

Can anyone point me to the exact sloka where it says shanta is related to dasaratha in valmiki ramayan

It was the joining of the raghu & the Soma dynasties:

Mother Santa is listed in the parampara in the Bhagavata-Purana
9.23.10

It was the joining of the raghu & the Soma dynasties:

Dasaratha's Children are listed along with Mother Santa in the Bhagavata-Purana

Well that's impressive.