Making memories

"You asked if you had to shave your head too."
This is what my mom tells me when I ask about my reaction to the news of my older sister needing brain surgery to remove a tumor.
You would think that I'd remember such a dramatic event like this happening, but I don't. In fact, I don't have a lot of memories on my own. Most of them are either patched together with other half memories or stories I was told. I rarely actually remember doing things in my past; and always feel that I have to qualify what I'm about to say with "my mom says that I did this..." Is this normal? Not to have one's own memory? Where does memory end and stories begin?
As I began writing more and more non-fiction, I found myself repeatedly calling my mom to verify if a memory was right, or more often, calling to find out what really happened. I could never rely on my own memory.
Conversational memory came easy to me, though; and sometimes I could even rehearse talks I had with various people verbatim. I remembered words. Maybe that's why I'm a writer.

This one I remember clearly. I started out asking about concrete details--making sure I had the 5 Ws correct. My mom tells me:
"It was the summer before your junior year. 2002. Erin had been complaining of reoccurring headaches every day, then they turned into migraines with nausea so bad she couldn't get out of bed. We got her home (she was away working at a summer camp) and into see Dr. Malkin. After studying the mri of her brain, he explained that the tumor, located in the back, was pushing her entire brain forward, increasing pressure against her forehead, and thus causing her migraines. She was in surgery in less than a week later and recovering at home just three days after her operation.

My sister was just in college when this happened; a sophomore, I think. I was seventeen. I should remember all of this, but I don't. Not any of it is my own memory. I remember my sister having this tumor, and praying for her once at my grandparents' church, but that's it.
My mom tells me next that my stepdad and younger brother offered to shave their heads in support of Erin. According to my mom, this terrified me more than my sister's cancer. This doesn't seem like me but I was seventeen, and a much different person than I am now. And why would my mom lie about a thing like that? I'm sure it did happen. I was confident that Erin would make it; she was my big sister, and a champion, and a fighter, so why did I have to shave my head, too? Wasn't Tim and Mike enough?
The truth is that no one shaved their heads, not even Erin, and she did come out of it just fine, well kind of. But the next thing I heard horrified me even more. She has no memory from before the surgery. My mom tells me her stories that she has. Sometimes I wish that I could just get rid of memories, have my mom make up my past. But this will never happen for me. I remember feelings and words if anything.
But I wonder what it would be like to have that kind of power.