Sorry, I don't - I didn't even know about Mabel being Agnes's sister. Unfortunately this side of my family is not close and was a mystery to me until this past year. I assume you've reached already on ancestry.com? I will add Mabel to my tree. Unfortunately since this sounds like quite a "family secret," we won't find anything publicly and I don't know anything. I'm sorry I can't help; I have a similar situation in my family where my great-grandmother and her sister were born after their named father had already died... out of wedlock that is, and I have no idea who their father is either. I wish I had more for you!