cliques in houses

They all have cliques. Think about it, there are 400+ girls in each house. It's impossible to be close friends with that many people. Think about any group of people ever - a class, an organization, etc. People naturally gravitate towards certain people. Same with sororities.
That being said, myself and everyone I know would probably agree that they have a best friend group within the chapter. It's like a niche within the niche. I can only speak for my house, but if none of my close friends were around and I needed help I would be comfortable with going to 90% of the other girls who I don't know as well in my chapter.
I'm sure some sororities are more divided than others in terms of people only really hanging around their main friend group, but the simple answer is that every house has cliques.