Hate being rude, but seriously, please learn how to use ellipsis properly (and sparingly), it's really hard to follow what you wrote.
I've no idea how you came upon the idea of "gay birth baby". Maiya is male, "Rin" is most certainly a female - no "gay birth" involved.
Also, i'm not sure who do you mean by each of your "his", but Maiya and Cedric CAN'T be brothers (again, one is isekai'd Japanese, another is resident of this world, no debate) so the conclusion is simple - Mayia is married to Cedric's sister, making them brothers-in-law, and Cedric - Mai's uncle.