Curse of the Black Widow

Curse of the Black Widow is a madefortelevision horror film, by Dan Curtis, and starring Anthony Franciosa, Donna Mills, Patty Duke, Vic Morrow, and June Allyson. It originally aired on ABC in 1977.

Higbie calls Laura and finds out Leigh is on her way. Laura tells him that Leigh is the one who was bitten, and Higbie tells her to get out as fast as she can. As Laura is in the midst of packing, she suddenly has violent convulsions and visions of two of the men who died. It turns out that Laura is really the killer. She has long suffered from multiple personality disorder Valerie is the expression of her feelings of inadequacy compared to Leigh. Valerie is also the only one who knows how to use Lauras curse. It turns out that Valerie has made several advances toward Leighs boyfriends when they turn her down, she turns into a spider and kills them. The only other people who know the secret are their mother June Lockhart and their former nanny, Olga Allyson. Their mother has been in a catatonic state since seeing Valerie kill Leighs fiance.Leigh arrives and finds her mother in a panic. Suddenly, Valerie walks in, taunts Leigh and her mother, and turns into a spider. Horrified, their mother falls out of the window to her death. Valerie encases Leigh in webbing. Olga arrives minutes later and realizes Laura has to die. She goes to an old farmhouse to find Laura cowering on a stall. Laura admits that Valerie killed their mother, and hurt Leigh as well. As Olga pulls out a pistol, Valerie takes over, turns into a spider and kills Olga. ........

Source: Wikipedia