In my opinion, Yes, it was. As you say, you feel the only way you can relate to others,or have a male friend is on a sexual level; because this was all you knew as a child from the only "friend" you really had. So sad. Did your parents see none of this?
My son and daughter were only 16 months apart, but they didn't have this issue-where did your brother learn this behavior? My ex molested our biological daughter, I found out later-when she was 13, and I took appropriate action--but brother and sister were
the only friends they had--and they were steadfast friends, he, to this day does not look at her in that way. They are 32 and 33 now.

Are you certain there wasn't some role model your brother was following? Some adult? Or other child around the same age doing the same things?

Sorry to be so inquisitive, but kids just aren't born that evil--and that was evil-JMO.
Sorry this happened to you--((((hugs)))) you are not alone