When I was a child, dses and I shared bath water, and then mum and dad shared bath water later on. This was because we had a very old immersion heater which took ages to heat the water, and also, I assume, because of expense.
I never liked having to use dsis’s bath water - I was the eldest, so I had to go second - but at least, when we were younger, I got to stay up a bit later than her, so it made sense for me to have the later bath, and I did feel there was an advantage for me.
But when we were both teenagers, and had the same bed time, I started to resent never having the clean bath water, whilst it was hot, and always being the one who had to clean out the bath and tidy the bathroom, so I asked my mum if dsis and I could take turns - we only had baths twice a week, so I suggested dsis go first on Sundays, and I go first on wednesdays.
She refused point blank, and went and dabbled her hand in dsis’s bath water and said it was still warm enough and not that dirty. I had the sense never to ask again.
Looking back, I think it was partly her favouring my sister, and partly that, if she had admitted it wasn’t fair for me to have to go second all the time, she might have had to consider letting my dad go first when they had their baths.