OMGolly!! My Heart is SO Heavy for You!! This is a
sad,
sad, very
sad story.
that being said,
I see NOTHING wrong in a Relationship developing between His Brother and You (if, and ONLY if ,YOU are comfortable with going in that direction).
Your Baby's Daddy would want You to be Happy, Cared For, and Treasured!! and If He could not provide those things for You HimSelf, I think He would be Happy that His Brother, His TWIN Brother, might take care of You and His Baby as He would have.