I'm still not convinced about the whole there were others OGB.
There's no talk of anyone getting involved with the Blight or the Grey Wardens in Andraste's familly.
Even the unknown alchemist , the only thing we got is , she may have worshipped the Old Gods
Dumat was killed in the Silent Plains , a bit far from Ferelden , any woman with a OGB in her belly would have come back obviously pregnant.
Looking at Hallisere death , it's possible she was attacked by a dragon , with the whole forest going up in flames.
Might explain why Andraste went against the Old Gods.
After reading bits and pieces in WOT 2 about Andraste , my overall feeling is ...most of the crazy theories are proved wrong , Andraste wasn't a OGB , Andraste wasn't a mage.We're back at the start , Andraste was a woman hearing voices.Maybe she was insane , maybe it was spirits/demons talking to her, maybe it was indeed the Maker.
Even the talk of causing cataclysm is thrown in the toilets , those "cataclysm" were caused by the darkspawn digging underground.And the Maker Chosen was actually losing the war against Tevinter.
Imho the whole sister tale is just there to throw us a bone and keep us speculating.