I think just reaching out and syaing you are available and support her decisions. IS really the only thing you can do here. Stay out of any negative gossiping, stay neutral, but she's family and she's the one you are sticking by.
Take her lead, but don't let her wallow too long in any negative conversations. Redirect and ask about future plans and find any excitement you can like helping her redecorate her new place, or ask her out for coffee/lunch/dinner/movie if you are local.
She's got a lot of sorting and rearranging to do. And while he's not dead, the dream of what she thought he was, where she thought they were going is dead and she will mourn that.