I think reaching out, itself, is hard to do/learn and wrestling with that issue because of the dream is going to be helpful to you.

In many ways the dream is just a dream so there was no issue with your father and cousin and that you didn't go back and get her in the dream is of no consequence in real life now. Do you still see or talk to your cousin? You could write her a note and tell her you dreamed about and were thinking of her (without telling her the dream) and maybe she could reassure you she is all right?