Dreams come from your subconscious, this can be from memories. Such as putting bits and pieces of things you have seen together and making something that has never happened before "real", such as the cousin, the room they are in, the pillow on the bed, the t-shirt he is wearing, the age either of them are all being separate things like cutting different pieces out of a magazine and making one picture out of them. So in turn the dream could be a mixture of any number of things she saw put together to form this nightmare, brought on by a rape scene in a movie, you guys talking about this other victims past, or any number of other stimulants.
Or a dream can be reliving an actual event that has happened in the past, being 4-5 years old when it happened would most likely lead to storing it away in the subconscious, where it is now breaking free. Personally I can hardly remember anything from before I was 8, only bits and pieces, but still very little, this may be my own suppression of something that I have yet to learn but may surface eventually.
If she is having nearly the same dream even if it isn't every night it still means something even though the activities in the dream itself differ. For most criminals, drug users, and rapists they tend not to be the one and done type. So if her cousin found an easy target, being the 4 year old she was at the time, logically he would continue to rape her. Not just once or twice unless the opportunity was extremely rare (such as living in different cities, parents asked him to babysit only once, or any number of different conditions that would prevent him from doing it again).
The room always being the same and the cousin always being the rapist leads me to believe that those pieces are burned into her memory from emotional trauma. Where the same thing doesn't happen every time in the dream, that makes me think she was abused multiple times in the same room, reliving each event on the different nights in her dreams or bits and pieces of each time it happened spread throughout the different nights.
The way I see it the following are the most logical
Either she can't remember these events actually happening and these dreams are the beginning of her realization and remembering it after your conversation or any number other memory aids towards the subject where I'm guessing you brought up a few details that hit home. This would explain the crying and her being silent after your conversation at dinner
Or she has known of it happening and is just now opening that part of herself up to you about it after she believed that you care enough to understand, would support her through it, and wouldn't run off because of it. This would also explain the crying after the dream, and the silence / contemplation after your conversation directly before you believe the dreams began. But if she had known of it, the dreams may have been present much earlier than just now.
Or what is least likely in my opinion it was all a weird coincidence and the consecutive nightmares were brought on from something possibly related to abuse/rape but not actually having anything to do with her cousin or her other than some sort of hatred for him ranging anywhere from he stole my bike when we were kids to he threw me off a cliff and tried to kill me, or no hatred towards him at all, it all just being a random coincidence. This would mean the silence at dinner was a coincidence and you grasped at straws that weren't there or it was unrelated to the current conversation, and the the crying was the result of a terrifying nightmare that seemed real but had no meaning.
In a relationship you have to be willing to tell each other anything and be willing to hear any number of horrific experiences, regardless of how hard it is to tell or hear. In even the best relationships you learn new things about your spouse years into it, and even till death. So if she is just now opening up about it, or just now beginning to relive it you have to be there for her through this completely understandable emotional breakdown.
In regards to confronting the cousin, I would consult your girlfriend first. If you say something to him and it isn't true he could ruin her reputation with the family completely by spreading what she said he did to her to everyone, this could also happen as a defense mechanism if he had done it so it would be hard to tell if he is lying or not. Also it sounds like he is much older than either of you and if he did get violent you may not get far in the conversation, that would show you his level of guilt and may even get him sent to jail depending if you press charges or have proof he did it, if you do go that route be smart, at the very least use your voice recorder on your phone when confronting him so anything that happens has a record, do not let him know he is being recorded, keep it in your pocket and let things play out as if no recorder was present.
If she just really doesn't want to talk about it, that usually means there is something to talk about and all of this wasn't some out of nowhere and now its gone experience. The hardest things to say can break you down to less than nothing, completely exposing yourself in the light of running makeup, snot bubbles, dry heaving from so much crying, and much more. So you have to show her that you are ready to see that side and comfort her through it. If she absolutely refuses to talk about it in depth or at all back up and wait till she is/seems ready.
Sorry for the long response, but this hit home a bit and any family member that abuses another shouldn't be able to walk a straight line once the bat of justice breaks their skull.
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