While we didn't have much to do with each other outside family Birthdays/Christmas, we have a great relationship, entirely due to how well she has always treated me
I only really noticed when DF first showed signs of really ageing. We worked together to support DM and get stuff done really well through his short illness and thereafter. It really felt like having someone who had my back.
Then, looking after out widowed mother there was a natural split with me doing finances and DSis getting landed with all the day to day stuff. I was happy to slip into my role as apprentice, doing anything she suggested without fuss. It seemed the best way to support her (and she wasn't confident with online finances anyway).
I love and feel loved.
Growing up, I completely missed how lucky I was that my six year older sister treated me so well, with so much love, more like a daughter than a sister.
It appears that I'm much more like my mother in personality and as a child, it was much too often me and DM being naughty together against my sensible elder sister.
Even so, I still managed to be jealous of the freedoms she was allowed (six years older, well obvs.) and ignored that I was allowed to do pretty much everything much earlier than she had been.
Parents pretty much treated us, and explained that they would treat us, equally, not exactly the same.
Then, when she had her own DC, I managed to be jealous of how her attention switched to her own children, when they came along, as if I was the trial run.
Horribly entitled child that I was, I just understood that having the full on attention of all adults around me was natural and only ever wanted to be independent.
Seeing her with her grandchildren I see how much she relishes the mother role and is so effortless good at it.