Richiev gave you a good starting point for your next revision, but best you study the information in the Training tab up on the top bar for logline conventions.

In answer to your question;

Your describing two separate inciting incidents – her discovering she’s pregnant and her discovering the mysterious beings. Which of these is the main one for the story as a whole?
I sounds as if her mother’s safty is her main concern, I therefore hold the need for her pregnancy in doubt as this inciting incidetn will require a separate goal and a separate sequence of actions. the story of her protecting her mother will remain unaffected if the preganancy is removed.

Lastly, “…mysterious…” is too vague a description for the antagonist. Best to describe them in a bit more detail – demons, gremlins or dead spirits. What ever you chose to use will describe the genre and will provide more information about the type of story/action in the film.

Best to specify an event that propels him into action, “?a murderous rampage?” is too vague in terms of time, place and character involvement.

How about:
After his best friend is killed by a serial killer, a boy must?
or to give it a greater degree of motivation:
After his brother is killed by a serial killer, a boy must…

Once he finds the sister, then what? It doesn’t feel like the story finished, as the danger still exists. Why not make the boy need to catch the killer to put an end to his rampage?
For example:
After his brother is killed by a serial killer his sister goes missing, a fearful boy must then catch the killer on his own if he is to save her.

The only thing I can’t understand is why must the boy find her/save her/do anything on his own? Why doesn’t he call the police?