All of these things, her younger brother didn't know, and she also didn't tell her younger brother about them.

So all theose years, the younger brother didn't know the truth about the abduction of his mom, the reason for committing suicide of his momhis mother's suicide and where his father was.

"Those years": this sounds better to me because we're looking back into the past, and it doesn't necessarily include the present. But I think some native speakers would be fine with "these."

And all of this iwas because the older sister wanted to protect her younger brother.

I would use the simple past since we were describing the brother's lack of knowledge as something that was in the past (though it may still exist).