I can't remember the gender of the main character.
I remember bits and pieces from different books, but I think they're all part of the same series. The pieces may not be in order or from multiple books.
Boy somehow ends up in a mirror universe that's darker and seems to have no people. He has to travel to a certain office(?) building and steal a book. I think this is towards the end of the book (I can't remember the events leading up to it). I think he finds the portal in a forest or something behind his school.
He goes into the ocean with mermaids or something and has to save the ocean from pollution.
Aliens exist and are observing Earth, because they think it's undeveloped and are waiting for them to be advanced enough.
His mother ends up in hospital and he does something to save her.
He goes to Ireland(?) and enters a mountain or something to find a race of people. I can't remember who showed him how, but I'm pretty sure she used magic.
He goes to Mars and saves a race from war with themselves(?). This place was hidden inside an illusion. He's also grouped with a linguist that learned their language.
He travels to another world where there's a prince. I can't remember, but I think it might've been part of a tournament.
His sister goes to a robot planet through a portal. I think the actual planet was a robot/AI and had lots of little robots on it. Can't remember what happens here, but he goes there and saves her from something.
It was set a few years ago, when computers weren't common (his family got one early in the first book, I think).
Thanks in advance if someone finds it.