Should i stop thinking about the fact that cancer is the reason i'm alive?

You're the cure to cancer.

That's pretty neat.

I think you might want to look into what would have actually happened if your sister's illness had got worse. It sounds like you believe that if that had happened, you would have been aborted and your fetal tissue somehow harvested to use on her. I suspect that what was actually intended was that she would have had a stem cell transplant using blood from your umbilical cord after you were born. This would have had no effect on you as an infant, and it could have saved her life. Since your sister didn't need it, it's possible the blood was used to treat someone else. So perhaps your birth means that some stranger who would have died is still alive and well today. Maybe that makes things even weirder for you, but life can be pretty weird.

https://www.anthonynolan.org/sites/default/files/AN%20Factsheet_CORDBLOOD_Fin1.1.pdf

I can understand you finding it odd that your existence is due to the fact that your sister became ill, but if you think about it, lots of people exist for even weirder reasons. Some people exist only because their mother decided to have one more mojito with the good-looking guy she happened to bump into at bar she'd never been to before. There's probably someone, somewhere who exists because her father decided to take an easy elective class in Japanese culture in his freshman year of university because he was a manga fan, and he ended up sitting next to a really cute girl who signed up for the course by mistake. And lots of people exist because their mother got distracted one morning and forgot to take her birth control pill.

Your conception was deliberate, so at least you know you were a wanted child. Some kids learn early on in life that they were unplanned and unwanted. That's really shitty, and growing up hearing that can totally fuck you up. However, I can understand you feeling conflicted since your parents thought they could use one of the products of your birth to help your sister. If you choose to look at it that way, you could think that you were just a necessary byproduct of the production of a kind of medicine to treat your sister.

Regardless of the reason for your conception, it seems to me that the most important thing is how your parents treated you as you were growing up. Hopefully, you never got the feeling that you were second-best compared to your sister or a "spare" that turned out to be unnecessary. If they really didn't want you and they were content with just having your sister, they could have put you up for adoption.

Wow! That's pretty deep, and heavy. I imagine you have many conflicting emotions that come up around your conception, and birth. I don't blame you.

Are you your sister's keeper?

Never heard about something like this before... you were meant to be born, so don't sweat it. At the time, the embryo or foetal tissue was made up of stem cells, which are undifferentiated cells. So, it wasn't you, until those cells became more specialised to actually form most of you, around 3-4 months. And at that time when you were actually brought into existence, you could not be used for any procedure. Therefore, I don't think you should dwell on it too much. We all have a lot to be grateful for in life, not just you, so enjoy it!

You can't stop thinking about something that had this effect on your life.

Reminds me of "My Sister's Keeper". That was a good read.