Hi Sherry -- Did a quick look-up in the Bowman/Congleton book, "The Ambrose Brewer Family." Your Frederick is listed as a son of Ambrose, marrying Lydia ? (you've supplied the Edwards!) The only children listed are Isara, who m. Nancy?; Frederick, m. Sara?' and Lydia, m. Sampson Williams. There is a note appended that says "Census records suggest that Frederick had three sons and four daughters". Sooo, your Susan and Dorcas may well be the other two daughters. Could you share any information that you have, particularly documentation?
As for Old Ambrose (to differentiate him from the many Ambrose's who followed) his wife was Mary Oma Richardson (there may have been another wife, possibly last name Russell. Ambrose is a descendant of George Brewer and Sarah Lanier, probably through their son, Howell.