Bless you. In my experience the best thing you can do is not be weird about talking about it. Ask about anything in the process you're curious about or don't understand. IVF is such an all consuming, obsessing, anxious and often uncomfortable process, and it can be hard if it's all you're able to think about and your friends and family aren't comfortable talking about it.
It sounds like she has a great relationship with your kids - I'd suggest just going with that (e.g. offer her the option of meeting up with or without kids, but don't proactively keep the kids from her because you are worried it will make her sad).
In chats about fertility stuff, concentrate on asking open-ended questions rather than suggestions ('have you thought about going gluten free / donor eggs / adoption / surrogacy, / fostering / getting a dog / acupuncture / exorcism / etc]. Forgive me if this sounds obvious but when feeling helpless even the kindest most sensible people can revert to advice and suggestions).
Accept that it is very possible she will not have children. Let her talk about this if she wants to, and try to resist the urge to tell her to be positive, or to be positive on her behalf.
Mainly just listen, just stick by her, and understand that you won't be able to cheer her up but she will remember your kindness and loyalty and openness and it will help her feel connected to the world.