My boyfriend introduced me to his sister as a friend and behaves so distant as if we are just a friend, not even close ones.

[spinstermanquee]

What marked me in your story is you two have been together now long enough, if he was really in love with you (and had honest intentions) he would shout it from the rooftops, no matter who was listening (like in the corny love story movies, haha).  A man in deep romantic love will attack every obstacle to be with his beloved, not erect cinder block walls in her path.

Please don't be ashamed, you didn't do anything wrong.  I hope you never find yourself in this unhappy space again... and that man who will shout his love for you from the rooftops?  He is out there!

[Batya33]

Agree with the others.  To be blunt it doesn't matter at all about the train wreck of the dysfunctional family.  You are not a priority to him.  You are not a family member, you are not treated as a potential family member-potential spouse.  It doesn't matter why and it's been long enough -more than- for you to be treated as a person he intends to marry in the not too distant future.  Don't indulge in all the analysis and drama.  Maybe tell him one more time in direct simple I statements.  "I see that you are not taking the steps to make good on your intention to marry me.  I don't feel as if I have a priority in your life the way a potential spouse should.  I want to be with a person who shows me that he loves and admires me as a serious partner including with his family.  You are not acting like that person."

[MissCanuck]

After 3 years he still has not revealed to his family that he has a girlfriend. 

And he's still making active efforts to hide your relationship. 

OP, it's time to read the writing on the wall. There is likely no future for you with him.