There’s nothing to say, and you shouldn’t say. Just act normal. That’s what she wants.
DON’T make her a Nebach. She’s not a Nebach. She’s just going through a very hard Nisayon.
Don’t ALWAYS talk about your kids, but don’t keep that subject taboo either. Kids exist in this world and that woman knows it. If you try to make believe that children don’t exist, then YOU ARE HURTING HER. She will realize.
It’s ok to tell her when you’re pregnant, and no, don’t leave it for your 7th month. Tell her early enough.
Don’t tell her what an “easy” and calm life she has without kids, or how much free time she has, or how hectic your life is with kids.
And of course, always be there with a listening ear and just listen and listen and cry along with her.