I'm in love with my brother but he's not my biological brother. How do we tell our parents?

My biological parents and I are British and they adopted my brother from the Ukraine when he was 9 and I was 6. We grew very close and quickly became best friends. We can always do everything together and always confide in each other. We have had very intimate feelings for each other since we were teenagers but kept it a secret. We have also had . He is 23 now and I'm 20. He really loves me and he says he wants to be with me forever and marry me. I'm in love with him and want to be with him too. I could never be closer to anyone else. But I don't know how to tell our parents about it. I need real advice. Our parents have always viewed us as just brother and sister.