Wow, this is like Christmas! (I just woke up and checked my email. I'm in Canada.) Thank you Sue and Cas and mckha489! So Ann was an Eshelby! And it looks like Edwin's mother was an Eshelby.
(Until now there were 3 possibilities re the connection between my great great granddad Edwin and his aunt and uncle William and Ann: 1. William was his maternal uncle, i.e. his mother was a Clegg too; 2. Ann was his maternal aunt, maiden surname u/k [Eshelby, sister of Mary Eshelby]; and 3. that Ann was his paternal aunt, maiden surname Rothwell. This narrows it down to no. 2)
The will you've found Sue is definitely the right one for Ann, for it notes that Edwin was in Kirkby Lonsdale, which is so. He'd moved there, the home of his wife Margaret and originally her father, to have a family. It's a very lovely town. Here's a fun vlog from there:
https://youtu.be/wrCf603-VeQ?si=wpaI_Sc4j_0NDx0e My great grandfather was born there. Sadly Edwin's wife Margaret passed there at @ age 31. I don't know if she died from childbirth.) So Ann had nephews Joseph Rothwell per her will and Alfred Rothwell, born in Rochdale, per the 1871 census, both brothers.
It looks very much like Ann was Mary's (Abraham Rothwell's wife's) sister. What a lovely headstone!
I note that John and Lucy Rothwell were witnesses to Abraham's wedding. I know that Edwin had a daughter named Lucy. Was Rothwell her maiden name? Was she the spouse or brother of John? (I'm not suggesting there's any likelihood that Lucy could be an Eshelby too, but it's not unheard of for 2 siblings to marry 2 siblings. It's happened in other branches of my tree.)
I'll check the census records to see whether Edwin sired an Abraham in honour of this man who was likely his father, and a Mary. My great grandfather was a Robert William Rothwell, Robert for his father-in-law I assume, and William for his uncle.
Cas, you mention that Edwin learned the plumbing trade from his uncle William and that Abraham and Mary Rothwell might've agreed that William and Ann Clegg raise him and train him (seeing as he and Ann didn't have other children, and might've been unable to.) The life of a plumber was very much better than that of the average local man in Lancashire in the 1840s and 50s, something Abraham and Mary likely appreciated. Work in the cotton mills was debilitating in fact, as you'd know better than I if you live in northern England.
Ann Clegg nee Eshelby hailed from Fewston just north of Leeds. Do the censuses indicate that Mary was from there as well? (My access to census records is inconsistent just now.)
Thank you again and again!!