This is a puzzling dream, so intriguing to work with. I'll need to ask some questions to pursue what your dream characters mean to you. The order of the scenes normally represents a dream-like psycho-logic.
Was there an initial setting for your ex's appearance BEFORE your deceased dad hugged him? (If so, what was there?)
How did your dad feel about your ex when he was alive? Was the bedroom you were in familiar? Or did it remind you of any bedroom you know? Being in a bedroom is an intimate place where he comes to see you (politely). The changes in his girlfriend are unusual elements, so they are meaningful. Was the first GF an accurate image of the woman he's moved in with? Or did she seem like someone else you've known or seen? If you imagined a character for her, what would her personality and behavior be like?
Now it appears that your attempt to introduce yourself causes her to transform--how was the new image DEFFERENT from the first one? And how might her imaginary personality change?
Did the interior of the house remind you of anywhere in particular? How did you feel there--familiar and/or comfortable? or unfamiliar and perhaps uncomfortable?
What would you imagine that your motivation could have been for following her to the further room? Curiosity? Territoriality? How did she change this time? How did her third image differ from the first and second?