I don't know, MWPP were only really nice to each other (as far as we can tell, obviously) and they turned out okay. The Trio/Twins are incredibly loyal to each other and yet can be ruthless and cruel to others.
Being nice to people of your own group is not exactly a sterling recommendation, but it's better than say, Voldemort (of course, so is most behaviour!) and it's not unprecedented in the HPverse when applied to other, more positively portrayed characters.