Are you certain this is the way they all feel about you? For various reasons, some of them simply may not be particularly close to your parents, and it would be easy for you to misinterpret their attitude and wrongly assume it was because of you.
But even if some of them look down on you (and your parents) because you were adopted, they -- and not you -- are in the wrong. Instead of looking down on you, they should be thankful that your mother and father have had the joy of being parents because of you! Sadly, they've allowed pride in their own family heritage to get in the way -- and that is sin. The Bible says that God "mocks proud mockers, but gives grace to the humble" (Proverbs 3:34).
Don't let the foolish attitude of a few of your relatives blind you to the love that most of them undoubtedly have for you. Instead, take time every day to thank God for your family. Nothing casts out hurt more than a thankful heart! The Bible says, "Everything God created is good, and nothing is to be rejected if it is received with thanksgiving" (1 Timothy 4:4).
Most of all, I pray that you will discover the greatest love of all -- and that is God's love for you. Commit your life to Jesus Christ -- and when you do God will adopt you as His child, and make you part of His family forever.