My favourite quotes from my favourite books

Siblings Alex and Robbie have lived with their grandparents for two years, after an incident that involved their parents occurred, and have often wondered why haven't they learned of their grandparents before from their mother...and what's with all the locked rooms in the mansion they're not allowed to go into, the locked shelf of books they're not allowed to read and the questions they're not allowed to ask. They find the answers to these questions one night, when their grandparents are out of the house and a mysterious being invades the mansion and attacks them. What are their grandparents' histories, who are these strange beings that keep showing up and trying to kill them, and what is the Otherworld that they keep mentioning?