Do you think Baba should've let out the secret to Amir before he died that Hassan was his half brother?

Yes, he should have, but it's no surprise that he didn't. Baba would have had his own reasons for what he did or did not say. It was his secret. None-the-less, he could have changed Amir's life had he fessed up sooner.

He should've but tell Amir not to tell other's in the Afghan Bay Area community or even in any Afghan community.