There's something similar between 'The Secret Garden' and 'Saint Seiya Lost Canvas', I guess? I Have read 'The Secret Garden'( a classic novel )by Frances Hodgson Burnett, and i just found some similar things with SS Lost Canvas.

1. There's a one girl and two boys that have strong friendship.
Secret Garden: Mary, Dickon, Colin.
Lost Canvas: Sasha, Tenma, Alone.

2. The girl have family relationship with one boy.
Secret Garden: Mary = Colin's cousin.
Lost Canvas: Sasha = Alone's little sister.

3. They are played on place that have plants and played important role.
Secret Garden: A secret garden, with rose plants, and swing boat.
Lost Canvas: A big tree, with wild flowers around it, on small hill.

4. There's a boy that always cheer.
Secret Garden: Dickon.
Lost Canvas: Tenma.

5. There's a boy that have a serious problem.
Secret Garden: Colin, he's diagnose not life for long time.The truth is he isn't.
Lost Canvas: Alone became Hades vessel.

6. The others two trying to help or save the one that have problem.
Secret Garden: Mary, Dickon.
Lost Canvas: Sasha, Tenma.

7. And always, always, there're so many disturbing to their friendship!
Secret Garden: Dr. Craven, Mrs Medlock.
Lost Canvas: Thanatos, Hypnos, and Pandora.

........Maybe......This is just my imagination, right?