My first though when I saw the 1881 census was that Minnie (9) was Mary A jnr's (30) child and not Mary Ann snr(55).
I must admit this rather dubious scenario did cross my mind and I'd often wondered why there was such a long childless gap between the two sisters. I think an application for a birth certificate is on the cards here.

With regard to 1901, I've not seen the census return with Mary & Mary but to be honest I've rather neglected them. I do Know that Mary jnr died in 1905 and My great grandmother Minnie had by then married my Great Grandfather and were living in Edinburgh at that time. She sadly died very young in 1903. As for the other sister Elizabeth, I lost track of her after the 1851 census and have thus far not attempted to trace her.
As for the boarders you mentioned in 1871 I don't recognize them as family but are we looking at potential fathers here

Curiously in the grave of Minnie, her mother and sister also lies a man by the name of Edgar clinch "Ridd", a man who's identity is a mystery
I thought this family tree stuff was meant to be a hobby, I think I should have taken up fishing instead

Your continued interest is appreciated
Regards
Stewart