My husband and I were in your position this past March. We met someone online, never in a million years imagined it to be more than a virtual fling, and then wound up falling in love with her.
You mentioned that your wife has had insecurities in the past. Speaking from my own experiences, even if these issues seem not to be in play so far with Miranda, they may crop up. You may say something - something not at all meant to be hurtful - that triggers one of those issues for Lissy.
Such was the case with my husband. He said something that really shouldn't have bothered me, but it triggered some bad memories from very early on in our relationship that caused me some doubts about myself (these things were almost entirely sexual insecurities).
Without knowing your wife, your girlfriend or the circumstances that surround her insecurities, all I can say is... be prepared to face that. You may have some of your own you don't expect, also. It's happened to my husband, though his have all been regarding our girlfriend independently of me (in other words, it's not really about me and her, just about her).
What I have learned in the six months I've been doing this is that it is extremely rewarding to open your heart to someone new... but it is not without its challenges, and they are very unique ones when it comes to three people. You are going from one relationship to four (you/Lissy, Lissy/Miranda, Miranda/Lissy and all of You). Each of those relationships need to be nurtured and allowed to grow, and always remember that the foundation for that All of You is the relationship you and your wife share. It needs to be as strong, if not stronger, or the other relationships will suffer.
I can honestly say that, despite the challenges and struggles, my relationship with my husband is better than it has ever been... and my love for him is deeper than it was before we met our girlfriend.
Even if our darkest moments, I am happy we did this... and believe it will work out. I also believe, though, that it will take time and effort to get us there... but then again, my husband and I have had an effortless relationship for the past six or seven years because of the work we put into it in the first three to four years.
Good luck to you all.