Joan Collins is, well, Joan Collins. She handles the role of femme fatale with her usual flair and drama. Ralph Bates [Dr Jekyll and Sister Hyde] fills Gino's shoes nicely as well. He's a classic British actor and handles the role with ease. Also good are Donald Pleasence [Halloween, Escape from New York] as the doctor who birthed the baby and Eileen Atkins [Wolf] as Gino's sister Albana who is also a nun. Pleasence is better in Halloween as Dr. Loomis but he handles the doctor role here just fine. Atkins is wonderful as the nun who saves the day in the end of the film. She has great expressive eyes that serve her well in this film.
Sharon's Baby is listed as a horror film and I'm sure it was written to instill terror in the hearts of 1970's filmgoers. However, it is best labeled these days as a horror/comedy. It is unintentionally funny but it's funny nonetheless. If you view this film in this light, you'll find it to be an enjoyable viewing experience.
For more info check out the film's entry in IMDB.