I think the quilt is beautiful. The seam that is off, must not be off much as I don't see it. Anyway, the love that you put into finishing that quilt for your sister is what counts. She will love it and so will her daughter.
It was so kind of you to do that. I will pray for your sister as I know about dementia. My mother had that, but she kept the beautiful smile thru the years before she passed.