Hi L, welcome back
Your son has loyalty, cares a lot for you, enough to confront his sister. But your daughter needs a lot of time alone to find your standing.
Your son, well, tell him to not include you in his relationship with his sister. Let them work themselves out. If she wants to go about her life without you it is her decision. She likely thinks her brother doesnt understand.
I know your pain. My youngest daughter came back in my life for 6 weeks and last week took off again, no explaination, cutting contact ties. Her loss. I've done nothing to deserve it but it is her choosing not mine. She decided not to communicate, to chat about it.
Be strong. If you feel happy to disclose all to your girlfriend thats ok, just limit the time on the topic and listen to her story of life. Build relationships that the other party wants building.