Hello @Dylonwarren1993..

I don't agree with some of the opinions already posted... I don't automatically see a reason to condemn your wife..

You version of thing is totally focused on the "suspicious" behavior of your wife... I mean... If she is with you she must like you, right? how is your relationship?
And, in regards to sisters and brothers bonds... Well, they can be pretty intense and expressed in particular ways.... Some people are bounded to their sisters and brothers in a very intense way... I see it very often that brothers and sisters hang around without clothes and even play with each other in a way that nobody else is allowed...

So, (I'm thinking).. she maybe have had some exploration with her sisters in the past (It happens when people are growing up)...
Do you have brothers and sisters?

I really wouldn't condemn your wife just because she has this special bound with her sisters... If You love her, I would instead try to talk to her and really try to understand this side of her with a mindset of acceptance...
On the other side, if you don't see a point in my way of seeing it, well, don't mind me... Its just an opinion...

I wish you all a wonderfull 2017!!