Re: Half vampire half mermaid hybrid
There are no hybrid mermaids and vampires in the game out of the box. While one parent can be a vampire and the other a mermaid, sims born in game of such a union are one or the other. They will either be a vampire or mermaid (and in rare cases, a human) and not have any traits, powers or weaknesses of the other. Reading the vampire lore books doesn't bestow anything other than knowledge really for sims other than vampires. To get sun resistance for a vampire, you have to build their powers to earn the perk which gives sun resistance.
But if you are getting mermaid children which are getting that buff or the burning effect and motive drain on the sim when out in the sun, then that might be a bug as it shouldn't be happening.
Hope this helps.