Heather is Cheryl, first rebirth. 
It gets complicated.....
Alessa is born of Dahlia. Father unknown, most people lean towards Kaufmann.
Alessa splits off baby Cheryl when Harry and his wife are visiting Silent Hill. I believe that Alessa saw the inherent good in Harry and created Cheryl from half of her own soul so that Harry would be able to stop Alessa's suffering in the future.
Cheryl First Birth goes home with Harry and his wife. Wife dies, Harry and CFB come back to Silent Hill for vacation. Events of V.1 transpire.
With the ending given to us as the right one by Konami in Lost Memories, Harry and Cheryl Rebirth leave the town.
Valtiel is the resurrector of the God, so he has a hand in the creation of Cheryl Rebirth. The ritual was not completed with Alessa/CFB brought back together, so he had to facilitate Cheryl Rebirth.
Harry took Cheryl Rebirth with him, renamed her Heather to try and stop the cult from finding Cheryl Rebirth and completing the ritual.
Then the events of V.3 take place. At the end, Heather goes back to being Cheryl Rebirth by name.
So, to answer your question, Dahlia had no other child to create Cheryl. While she may have had another child, it wasn't Cheryl.
It gets complicated.....
Alessa is born of Dahlia. Father unknown, most people lean towards Kaufmann.
Alessa splits off baby Cheryl when Harry and his wife are visiting Silent Hill. I believe that Alessa saw the inherent good in Harry and created Cheryl from half of her own soul so that Harry would be able to stop Alessa's suffering in the future.
Cheryl First Birth goes home with Harry and his wife. Wife dies, Harry and CFB come back to Silent Hill for vacation. Events of V.1 transpire.
With the ending given to us as the right one by Konami in Lost Memories, Harry and Cheryl Rebirth leave the town.
Valtiel is the resurrector of the God, so he has a hand in the creation of Cheryl Rebirth. The ritual was not completed with Alessa/CFB brought back together, so he had to facilitate Cheryl Rebirth.
Harry took Cheryl Rebirth with him, renamed her Heather to try and stop the cult from finding Cheryl Rebirth and completing the ritual.
Then the events of V.3 take place. At the end, Heather goes back to being Cheryl Rebirth by name.
So, to answer your question, Dahlia had no other child to create Cheryl. While she may have had another child, it wasn't Cheryl.
