I really really need some advise, im confused and dont know what

I was adopted when i was six months old, i have grown up knowing my biological mother, but i did not know who my father was. i grew up with a realationship with my mother too. I love the family that i was adopted in to, i love them with all my heart, but it has it's prob, ne ways, me and my half sister (both adopeted in to the same family) fight all the time. recetly she has been saying oh we arent real sisters cuz i have a different father than you, and that hurt deeply i cried many nights to that. because of that my bio mom start to look for my bio dad. she found him, but it turned out not to be that guy (we knew because of a condition that i have called essential tremors. (which is nothing more than the shakes) well it is passed on though generations.) and no one in his family had it. so my mom began to look again. and she found him, and he lives about an hr from me. i have started to talk to him and i have seen him twice, (my parents know that i know my mom, but they dont know that i know my dad) so everything has to be secert. i feel really bad about sneeking around behind thier backs, but i dont know how to tell them. they are already really upset that i have a realationship with my mom, they think that i will go off with her as soon as a i can, but i wont i love them too much. i really wanna tell them, but i jus dont know how, it makes me so sad to be keepin a secert like this from them, i feel like i'm living a double life, that i'm living in two different households. i dont know, i cry all the time, when ever i talk to him, i jus dont know i love all of them and i really wanna tell my parents but i dont know how to. I have told my bio parents about how i feel and all and they understand that i have told them that i really wanna tell my parent, but they understand that i cant, and i wanna tell them, but i cant, i dont know what to do. Please help

hey i know its not the same but i have a half brother and i didnt know untill i was 7. he comes to see me as much as he can. I am sure if u told your parents they would understand, they know how much u love them. u know they might not be your bio parents but they are your parents and they have taken care of u and i am sure u look at them as your parents and u love them very much. Tell them that nothin can change how much u love them and u always will. i dont know if i was any help but if u ever just want to talk u can e mail me at brittany_broome@hotmail.com or add me to msn
xoxoxo

Yea see, they get pissed off when i talk about my bio mom, and like they hate when i ask if i can do thing with her, they truely think that im goin to leave them and im not i have told them that i wont so many times, and they jus dont get, i really dont know what to do, cuz like i dont wanna start shit with them, but like i wanna know my parents and they jus dont see that i think of my bio parents as friends i have told them that so many times and they jus dont see. i getting so tired of it, i dont know what to do.