Lissy has anger management issues. Only she can decide when and how to address them, assuming she doesn't eventually get a court order against her taking the matter out of her hands. I hope for everyone's sake she will learn the necessary coping skills sooner rather than later.
With all due respect, Darkhorse, you have bigger fish to fry than the insecurity of your lovers. The much more pressing challenge, IMO, is developing strategies to effectively deal with Lissy's anger problem.
But that is not the problem you have asked for help with. In Lissy's case the insecurity issue is tied to the anger/self-control issue; working on the bigger problem should help solve the smaller problem. (And Lissy is correct: her failure to control her rage has negatively effected her life and will, in time, damage or destroy much of what is good in it. )
To help you positively address Miranda's insecurity issues, keep talking to her and demonstrating to her how important she is to you and Lissy. Talk, and talk, and talk some more, and does forget that actions speak louder than words.
Best of luck to you all as you find your way in all this.