If they can have children in the first place then the risk is worth it already. Besides it's only a 25% chance of having a werewolf-mermaid kid.

If they were half-youkai, I'd say half of this might be plausible under the series' existing logic.

On matters of technicality, if they gave birth to the second theoretical child, it would proudly be the most horrifying youkai in Gensokyo short of some sort of spider-bear-tiger youkai.

Heh...the "Accident!" reminds me of part of Fatal Fury 3.