The first thing that stands out to me is that if your relationship is so unstable that you fear she might leave you for someone else, then the last thing you need to do is try a poly relationship. Poly will not fix anything.
My advice would be to work on communication and building trust in your relationship first and foremost.
And... no one can "steal" a person... it's a choice that person makes whether they will walk or not.
What is it that you think he's offering her that you aren't?
Pinky, 41, f, bi. Married to Bear, 42, m, straight.
Not dating anyone right now.
I became insane with long intervals of horrible sanity.--EAP