Further info on that 1891 census has
David Cummings Head age 27 bn Edinburgh
Isabella (sister) 26 bn Edinburgh
Georgina Niece bn Shotts
Robert Latta (Boarder) bn Rutherglen
Isabella certainly had a brother David as per above, and Georgina's relationship to him and her birthplace, which is correct as per the image of her birth cert and same address in 1891 census,so a good starting point.
We know as from a previous post when and where David was born and his parents, Bernard Cummings & Ann McCallum.
Bernard, David's father died in 1865. We have found David & Georgina in 1871 as per previous post.
Now the tricky bit. Isabella (BC 1865/66). Either her mother Ann(McCallum) was pregnent with her, or Ann fell for Isabella after Bernard's death. I and other cannot find a birth registration for her, either with surname Cummings/ McCallum.
I have no doubts that Isabella is the sister to David.
Do you know per chance when Isabella died.
SS