This is a new information for me. I have read Ramayana twice but I never came across such a thing. May be it is in Sanskrit and I don't understand all the meanings. As far I know, King Dashartha did not have any kids, even though he had 3 wives. There for he worshiped Lord Vishnu and accordingly he got a boon. A sweet dish was given to him and if his wives ate it, they will become pregnant. Thus he gave it to Kausalya, his first wife. She gave half of it to Kaykey, the second wife. Both of them gave half of the dish to Saumithra, the third wife. Thus all three became pregnant.
Sri Ram was born to Kausalya
Bharath was born to Kaykey
Lakshman & Shathruknan was born to Sumithra
But no where a girl was born. As the ISC members said if a girl was born to Kausalya earlier, then why did not she have a second child. I have no right to question epics, but it is just my doubt. And also if that was the case, when King Dasharatha got a boon, he would have asked Lord Vishnu to cure his daughter's illness rather than asking for something else.
Some how for me it is not digestable.
"Do not give up, things might not favour you always"