You can't do anything legally to make her see the boys. There is free will in this world. Even if there was a way of legally enforcing it, she would only resent it which wouldn't be any good for your boys.
I speak from experience that being non-custodial severely damages the relationship, unless you have some kind of co-parenting arrangement.
The less you are there, the less it bothers you. Absence does not make the heart grow fond, it makes it grow distant.
She's left and has her own life - she's not there as much because she doesn't want to be. Tough but true. You are separated and unless you get back together, you cannot even begin to make comments on her love of those boys (you can't know what she feels) or the amount of visitation she has.
That is as controlling as my H saying all the times I can't see my children and it's a tad patronizing.
Take it from me - leave the issue alone or you may put her R's with them under even more strain.
