Doubtful they are twins... one of the "sisters" is good looking with perfect tits, the other is kinda ugly with small tits. Similar hair, a slight resemblance maybe, but not identical twins by far.
I know exactly what you mean. I think it's that it let's us know that the feelings and urges they have are authentic and it's not simply a matter of performing for whatever reason (money, fame, etc). That you can appreciate the very same thing is a truly encouraging aspect of it.