Hi
I am an adoptee from the 1970s. I have had contact with my birth father (now deceased) who was also adopted. So, I have had the experience of being both someone seeking a family and the 'found' person.
These are just a few thoughts.
It is possible that your cousin doesn't know they are adopted. I know that sounds crazy, but you cannot rule this out completely. It's easy to forget how different things were when closed adoptions were the norm and I wouldn't assume anything about a person born in this era.
I would also urge you to be cautious about what you think you may or may not know about contact.
You do not know they didn't try to contact your Auntie, and she didn't reply at the time, for whatever reason, or the message wasn't communicated to her in the right way if it was passed to her via an intermediary, or didn't reach her, and it might have presented as a rejection to the adoptee.
You don't know if they actually have had some contact that you are not aware of. It is not uncommon for birth mothers to say, "I will tell my family eventually." But the birth child then gives up hope.
There are a host of reasons why your Auntie may not have been able to be transparent about this.
None of this is a criticism, it's just something to consider. If you have more questions, I'd be happy to discuss in a PM about my experience with BM.
In terms of honouring your auntie, the cousin may not be interested or may want to know but might just be curious and not looking for a long-term relationship. This could sting.
All of this said, because you are a cousin and not one of the birth parents you could be at a real advantage. If the adoptive parents are still alive, they are likely to be sensitive - even now - about all of this. They went through this experience very different to the adopters on this board today. However, they are unlikely to find a cousin very threatening and it could all go very smoothly.
You may be heartened to hear I am in touch with my birth cousins. It just isn't intense and fraught in the way it was with my BF and it's really nice to be connected.
I hope it goes well for you.
I know it might present as negative - it's meant to be more to help you avoid a 'I wish I'd considered that' scenario.