A 12 year old human/alien hybrid, struggling to fit in on his alien planet, discovers he looks exactly like his fully human ‘half brother’ on earth and leaves his planet with the mission to be adopted by his human family. (ET meets The Parent Trap)

This sounds like a good idea for a coming of age.

I assume that one of the boy’s parents is human and the other alien thus creating a genetic hybrid of the two. If so you can use this to personalize the story a bit by giving him the goal of finding a lost parent as appose to getting adopted and describe a road trip (every body loves a good road trip movie) only across the galaxy.

The logline is too wordy and is not structured to maximize the effect on the reader.

Try describing the boy more efficiently then state the inciting incident and then describe a clearer goal.
e.g:

After a half human half alien boy, a miss fit on his home planet, discovers humans look like him he travels across the Galaxy in search of his human mother.

Or something to that effect.

Hope this helps.