Can two sisters share the same best friend?

I have two friends who I met in grade school who just so happen to be sisters. I met one in my P.E. class first. I met the other at the neighborhood playground. They did not attend the same school. We are all friends still to this day.
But lately, the sisters have been bumping heads lately (as they live together) and when they do, my name always tends to come up.
They bicker about who is more of my best friend out of the two, whom met whom first, the most things in common.... (Childish, much!)
I love them both the same. They both have their flaws, but I don't look at one of them to be better than the next. I feel it causes unnecessary confusion and also don't see why it really matters. Then the worst part is they'll tell me about it like they are looking for me to tell them who's the best friend, and I truly don't have an answer so I just kind of laugh it off because in my eyes, it's petty.
Is this normal? Anyone ever been in this sort of situation? What should I do to stop this? I don't wanna ditch them but I hate to be bought up in a negative situation and sometimes feel like I am the cause of the turmoil; although I know I am not. What are your thoughts please? Thank you!