Sarah Ross is also my grandmother. According to my mother (Marion Shaw - Sarah's first child) her mother was indeed a foundling and was taken into care. She was later fostered twice: by the Gribben family and the Conroy family. I don't know the timelines here, or which family was the first to foster her. Apparently, she had a dreadful childhood, being treated predominantly as a slave, doing farm work and household duties. I hope this is of some help to you.
I would also like to know who your parents are, so that I can place you in the family tree. Love to keep in touch.