First and foremost, I’m sorry for your loss, anonymous.
This can be a difficult time for you and your talonflame, but there may be a few things you can do. First, ensure that her nest is in a place free from stressors. Keep her space clean and quiet, and always stock it with plenty of food and water, as well as access to fresh air. The more comfortable she is, the easier it will be to keep her calm.
Next, tend to her needs personally. The calmer she is, the easier it will be to go through the rest of the reintroduction process. Feed her treats, give her wing massages, and reassure her that you care deeply for her until she seems completely at ease.
Then, reintroduce her chicks to her nest. Depending on how violently she rejects them, it may be best to do this step slowly, with one chick at a time. Place the chick in the nest and watch for her reaction. If she doesn’t toss it out (and I mean that literally, so be prepared to catch fletchling here or there), add the next one in until the clutch is in her care.
You’ll need to follow this up with a combination of regular check-ups to the pokémon center (which may also offer help for her mental health and assistance with the grief process) and active training, the latter of which should be focused on maternal tasks. That is to say, you may need to teach her how to be a mother again.
Above everything else, though, remember that this may take time. Losing a child is almost always heartbreaking for a mother, even one that isn’t human, and it may be difficult for her to heal. Be patient as you help her, and eventually, she may come to love her surviving fletchling.
If all else fails, though, be ready with a foster parent, or another pokémon who’s willing to act as their mother. As sad as it is to say this, sometimes, a mother who rejects her clutch never reaccepts it.
Best of luck, anonymous!