Trapped inside their family cottage, a group of detached siblings must battle a vengeful witch and break a centuries old curse on their family.

This sounds like a great horror premise, a containd genre film is what many producers are looking for, and this sounds like one. The logline, as mentioned above, does need some work though.

You've mentioned multiple protagonists, which can be done but will work against you in the long run. You're far better off specifying one. Can you define one main character for the story? Can you define the event that starts the story for this character? Could it be that one of his or her siblings is killed by the witch? The subsequent goal could be to kill the witch and it needs to be specified in the logline as well.

Good advice. Another angle could include a deadline that they have to complete this by or else something worse happens

I think Dk's advice is strong.