Is it normal for family to be like this?

I have never met my father, nor do I know anything about him whatsoever except his height. I have always asked my siblings who he was and how he was with them but always seem to change the subject or simply just avoid it. My oldest sibling is 33 out of 7 including me. My youngest sister is 21 and is older and adopted but she knows her real parents. My next sister after her is a few years older and apparently has a different father than the rest of us. To me, that's believable because she's the tallest, but what really screws everything over is that my oldest sister has gone to Mexico several years back to meet her father alog with her 2 kids which were already at their adolescent ages bak then. Since I "know" that the rest of us have the same father, I can't really tell if she also met MY father when she met hers. Did they really meet my dad under my nose? I really don't know what to believe. Either my family has been saying lies about my real father to me or they're seeing him without me knowing and lying about everything else.
What options do I have in knowing who and where my father is?
It is normal is keep living like this and family to screw up my mind about him?