Fantasy book series about a teenage boy who develops powers and can enter another world

Question

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.

Answer 1

A possibility: Skellig, a children's novel by David Almond, published in 1998 whose protagonist is a 10-year-old boy. It has the elements of winged beings, angel-like mentor, sister whose life is in danger, although not entering another world while asleep. Quoting from Wikipedia's plot summary:

Michael asks about arthritis and how to cure it, talking to doctors and patients in the hospital where his baby sister is being treated. Grace, an old woman, took a run through the hospital and came to see him. Once Michael returns to the hospital, he finds out that Grace has died. The creature whom Michael had moved from the garage—revealing a pair of wings at his shoulders—introduces himself as "Skellig" to Michael and Mina.

Michael's baby sister comes dangerously close to death, necessitating heart surgery. His mother goes to the hospital to stay with the baby and, that night, "dreams" of seeing Skellig come in, pick the baby up, and hold it high in the air, saving her. He subsequently moves from the garage after saying goodbye to Michael and Mina, answering their questions about his nature by saying that he is 'something,' combining aspects of human, owl and angel.