I'm looking for a book series that I read at least 10-15 years ago. It was definitely a series as I finished one book but lost track of it whilst waiting for the next book to come out. I'm sorry, I only have snippets of details and some details may even have become mixed up.
The main protagonist is a teenage boy and the story takes place on Earth. The main characters are humans. The basic premise is that he develops powers and enters another world when he is asleep but possibly then learns how to access this other world whilst awake. He has powers and there are the typical bad guys who are responsible for the loss of his sister. However, he has a mentor, which I feel is almost like an angel. The beings in the other world have "wings", and at the end of the last book I read, the protagonist gets his "wings" (literal or figurative, can't quite remember) after facing a jury as a result of something he did wrong.
The protagonist's sister is killed early on and there is a flashback scene where they are at a garden waiting for a rare flower to bloom and he loses track of his sister.