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I think it would be pretty obvious if you were adopted. Just look in the mirror and see if you look like your mom and dad or not. If you don't then you should ask your parents why they never told you and any other questions you'd have.

"So for a bit of context I have a brother, Lets call him jeff. Jeff recently got a girl friend. Our parents and his parents have gotten along very well"

that confuses the hell out of me.

but that aside, how old is 'Jeff'? that they wouldnt show you the birth certificate when its brought up is terrible. it just drives further suspicion. it sounds like it could be due to adoption to me too though. if youre questioning it then youre really at a point where you deserve to know one way or the other. if you can have a mature conversation with your parents i would.

"Our parents and his parents have gotten along very well"

Confused about this part. "our parents" = You and Jeff's parents, "his parents" = ???

well you could have either

been born in a different country than you thought

was born a different sex

have a totatlly different birth name

those are some things that popped into my head....though they are ridiculously implausible, and most of those are stuff that only happens in comedies :P. other than that adoption is also something...though I don't see how they expected to keep it a secret forever. and it might not just be you, the adopted one might be jeff...but then why would they hide yours too....really is weird

does it matter if ur adopted or not?Im sure ur parents love u...isn't that all u need?

not really a problem if u were adopted.. u are lucky enough to find a family when many kids out there needs one

They love you (probably since they raised you but I am not them)
They knew you were not their own flesh and blood but raised you regardless.
Now that you're all grown up, you repay their love with fear.
Fear of what? That you are not biologically related?
That's is just a tiny aspect of who you are. Your biological parents don't define who you are. (It is important to know health wise)
If you learned that Hitler was your great, great grandfather, you don't turn into a Nazi. (although if you always were, that would explain it)
Blood may connect people and sometimes you can't get a say in it but those two chose to have a connection with you.
Maybe they didn't tell you guys in fear, fear that you guys won't love them the same anymore and it seem it wasn't baseless fear.
Will being adopted make them worse parents? Will your childhood suddenly change for the worse since you found out?
No, those things have been done and past. Be proud that your parents can love a child that is not their own.
But, I have never been in your situation so I wouldn't know.