Like me and my sister, just a normal attitude between people that are really family. Why take it so seriously?

I feel like if it was actually serious, they wouldn't be bringing it up publicly. If they feel comfortable sharing, then they probably aren't on the same level of extreme you might think they are. Also, some people come from families with more physical backgrounds. (not in a creepy or bad way) What I mean is they might smack or hit each other, not THAT hard, but a lot of the time one or the other will shrug it off. Taking this seriously is like sexualizing skinship. Don't take it to another level! A butt slap or head whack is not a big deal between siblings that grow up in families like that, why should it be a big deal between these two when they both admit they do it and don't care?
Also, you are right. They ARE full grown women. But remember that most full-grown women don't live with each other as closely as siblings do. I would say the environment here is more similar to a siblings relationship than a grown woman's because they are together so often.
If you still think that it's immature than I don't know where you're coming from. Sure, maybe their relationship could be better, but some people have their own way of getting along. If it has worked for them since they debuted, it will probably continue to do so.