Real World Seduction was my first favorite book and thought it was severely overlooked, it is probably the best book I've read for techniques. It doesn't go over (from what I remember) inner game but it works extremely well as a complimentary book after a person gets their inner game fixed up.
Anyways from my experience the reason why people tell other people to treat women as if they were their little brother or sister is because it basically all in one point sets aside a lot of explaining. How do you treat your younger brothers and/or sisters? Most of the time you're in command. You don't have to hesitate or watch what you have to say to them, you just don't care, you could say the stupidest thing in the world and not give a sh1t what their opinion is. When you're in command a lot of things come naturally such as being funny, being a male every woman desires, etc.
What also makes it easy telling men to treat women like their younger brother or sister is that they will already have experience with it. When men are given the advice to approach women, be ****y and funny, at first they won't have much experience with it and will be struck with a swarm of butterflies in their stomach. In fact the first couple times men probably won't even do it! When told to treat women like a younger sibling, they would already have experience with it making the process easier on them.
Although while I said all of that, it actually depends on how a given person does treat their younger sibling. Some treat their brother or sister like an AFC would with a girl or in the very rare instances, the younger sibling is the dominant one.