Thanks Gadget and Carol, your insights have been very helpful. I totally agree the gent and Minnie at the back definitely look alike, so am certain they are brother and sister. Maybe the woman seated is his bride, and not Minnie's sister Nellie. If you ladies are good at facial recognition, here is a photo of Nellie(L) and Millie (R) a number of years later. I personally can see a resemblance between Nellie in the wedding shot and the older Nellie is this shot though.