If they're actually blood related cousins, what she's suggesting isn't actually a good thing due to recessive genes. Rudeus's father was a dumbass for not letting him stay with and marry his childhood friend, who was definitely not blood related, and even partially non-human. That would've potentially allowed for the "hybrid vigor" (heterosis) effect to kick in.
Also, this manga's getting way too dark. I noticed that a lot of stories tend to use that as a crutch if they drag on too long. I wonder if it gets difficult to write for and draw so many different characters, so the author just decides to kill off the ones they can't think of ideas for. Which is bad, because it really takes away a lot of the life and communal feeling of the story and just makes it a one-note drama that relies on unengaging cliches that feel forced. Plus, it makes the story a downer if it's all negative all the time and you don't feel good after reading it.