One of the most dangerous things a boyfriend can do is try to wedge his way between sisters. See, sisters are some of the most protective creatures on earth, right next to mother rhinoceroses. They have another sense where the crap you're about to pull is suddenly on their radar; it's as if they can see right through you. It's no secret to them that you're about to try to weasel your way into their position as a confidant, a fashion adviser, their support system, and most importantly, sister. This is when the defenses are raised and the arsenal is prepared. It's just something that you shouldn't try to do, yet it still happens. I can guarantee that this doesn't happen without casualties, though.
When the sister or sisters can see what the boyfriend is trying to do, the small amount of respect for said boyfriend is gone. It's only natural for someone to hate another's guts when they try to replace that person. There should be no "try." It should be up to that person to decide whether their sister or sisters are enough. There should be a separate space in the heart for sisters and boyfriends because let's be honest, there is definitely enough room for the both of them. The fight for the sister's heart is pointless. If the boyfriend is not sneaky with this replacement, which they never are, I think he doesn't deserve respect from the sister or sisters.
If things go well between the boyfriend and the sister, the boyfriend will eventually be the one who consumes most of the sister's heart when they get married. That's all fine and dandy, but the sister will always be the one who gets the unfiltered, plain truth no matter what. There will probably be a suppressing of thoughts or opinions at times in the relationship between the sister and her significant other, but there will always be an openness that sisters share because they are stuck with each other. They're family. They've seen each other at their worst. They've spilled all the dirt. There is no confession that is a surprise between sisters. This means that if a sister has ever been wronged by a guy, her sisters have heard about it. This is just another weapon added to their arsenal, and sisters don't forget stuff like that.
So, men, take this as a warning. Don't try to replace your girl's sisters. This will only make your life terrible. If you think you have successfully done this, think again. The sister will eventually realize that she needs her sisters because they fill the massive void that you leave.