It doesn't seem fair that you and he don't get any quality time alone when you say you've all put the one act of betrayal behind you.
Are you in a triad (the three of you have a relationship together) or a V (he has a separate relationship with each of you)? Do you spend time alone with her? Have you tried speaking to her about this one on one?
If you are not getting what you need from this relationship it feels more like you're being used to enhance what they have rather than being treated as a separate person with needs and desires of your own. You're being almost forced into the "unicorn" role to be with him when you may not wish to be their unicorn.
I'd rather be hated for who I am than loved for who I am not.