Need Help With Jealous Sibling?

I am expecting a baby with my spousse who has a JEALOUS daughter who is twelve. She is posting and writing and recruiting help on HOW TO KILL ME AND OUR BABY ( who is due in MAY ) She WRITES all over " I want that baby dead and her gone " She has been writing that her dad needs to compensate her with money and shes thinking of ways " how to get money " out of him. My spouse is afraid that " she will hate hiom " if he confronts her..he lets all these death threats go, she steals my personal belongings..and acts like she is the victim..HELP>.I dont know what to do..I am afraid she will hurt my baby..Her mother has mental health issues...they say its hereditary..I think she is very unbalanced..My spouse is ignorong it,,I think it needs to be delt with.he is blaming ME for her being like this...I dont know what to do..HELP!!