Does your wife have any other people in her life she enjoys intimacy with? I'm not talking about sexual intimacy here, but the pure content feeling of being close to someone and happy about that. If so, that might help her appreaciate that she is not in any way redundant and deficient - that she is an unique individual and loved for who she is.
Sexual intimacy for many people though, at least for most mono people, is something unique and no amount of "but you love both of our children don't you?" will make them change how they feel, and I'm not sure if they need to. It's about deciding what level of discomfort we are comfortable to live with on a day-to-day basis. All of my relationships have varied amounts of insecurities around "what if they secrectly want to be with someone else more than they want to be with me", and they tend to fluctuate with time (I'm most insecure when there are other life stressors either in my own life or in the relationship I'm getting the insecurity attack about).
Oh and btw, plenty of poly people struggle with insecurity too. That's a part of what fuels the whole OPP/veto/hierarchy debates. This is not some problem your wife has that she needs to get over alone. I frequently get desperation attacks about my partner having others, despite me having other partners myself! There's no shame in being human and in reacting in human ways.
Me: bi female in my twenties