Should I Be Okay With My Twin Sister And My Husband Spending Time Together Behind My Back… Am I The Only One Who Thinks This Is Wrong?

I almost had myself convinced that I was losing my mind. I've been watching my twin sister and my husband's behavior change whenever they are around each other for a while now. I've made comments about it and that's always started huge arguments that lead to nowhere so I started doing my own little investigation into the whole situation... and found that I am not just seeing too much into it. My sister got another phone number and they talk often... they meet up in hotel rooms and here at my home etc. So whenever I see them pull up in the driveway together in our truck I lose my mind. They can't understand why I'm so upset and now I'm am being treated as if I'm crazy because I won't just let it roll off. Apparently I am expected to have greeted them with a smile on my face.