I slept with my best friend's baby brother.
And she's unaware of this?
She doesn't know, but I think if I tell her this story we'd be much better friends!
I'm a bit confused as to why you think telling your friend you slept with her younger brother would make you better friends. Most people would be upset if they found out their friend had sex with their sibling.
Well, I think I'm pregnant from it.
This sounds like a very complicated situation. If you think you're pregnant, you should first talk to your friend's brother so he knows that he may be a father. You should also take a pregnancy test so you can find out if you really are pregnant.