I think having secret babies with cousins is pretty much a deal breaker and would be a deal breaker for many people. I’d say cousin’s wife needs to know. I’d not pretend like it’s all fine and dandy and act clueless going to his house. It’s helping him and your wife to continue their deceiving ways. Also do you have documentation of his actually being a donor? Didn’t they ask her any questions in the clinic? Did you think it was your sperm? None of it makes sense