Ha I actually made an iOS & watchOS app based on that side of the equation after thinking "what's the minimum input I can provide for good hydration reminders?"
It gives a reminder to drink water any time it's been three hours since you've peed...
Not super scientific, but less of a hassle than tracking water input and it tends to adapt well to factor in things like exercise where more water is needed. It also tends to even out over time when drinking alcohol or coffee. Urine color can also vary depending on foods, like how B12 makes it fluorescent yellow, so the time factor can be another useful indicator.
No offense to your sister, but I've heard a lot of crap prefaced with "$PERSON works in $FIELD, and they said...". Her suggestion is demonstrably one of those things, as other data [and, no, I'm not finding a goddamn citation] imply that there's little basis for her suggestion.
Yeah man, my boss is a programmer and doesn't see any reason why he shouldn't use global variables any time he feels like it. That doesn't mean he's right!
Let's assume that's the consumption needed to optimize the functioning of the renal system. But is that necessarily optimal consumption for the whole body?
With no offense to your sister - who I'm sure is correct - hypothetically, maybe this could result in some other condition that a urologist might know nothing about.
It works most of the time, but it doesn't work if you are drinking alcohol... your body has to 'waste' water to flush it out of your system. There are probably other drugs and medications that have the same effect (and no, caffein is not one of them, at least to my knowledge).
