Its hard to say how much she understands the limit on her sleeping with her boyfriend alone. Have you discussed your feelings with her? Have you asked her what she feels? Does she have one-on-one sex with you? If not, then it's likely she is craving the intimacy of one-on-one sex. I'm guessing you and your husband still have sex without her. Threesomes are great but...not for every time, if you know what I mean? That sense of connection can get lost with three in the mix.
What, exactly, do you think will happen if they have sex alone? What runs into your head when you picture that? Do you think he will suddenly decide to leave you because of it?
Give yourself a deadline. Send them out on a date together. Let it happen.
At two months it's time to take a deep breath and start building individual relationships.Each set of two needs to work on independence. A triad without strong individual dyads is going nowhere.