I don't usually come up with strange conspiracy theories like this one but if you hear me out I believe this one may have promise although I am very bad at explain these things.
My theory is that Ace has a twin sister called Ann, mainly because I believe that every single thing that was said in the flashback has deep meaning (Which is very much like Oda).
In the Portgaz D. Rouge flashback she said "If it's a girl I’ll call it Anne" which has made me think for awhile maybe there is more to it especially since the scene was shrouded in mystery and like we know from the "Brother Vow" flashback not all the pieces of the puzzle were there (Sabo)
You are now allowed to call me a nut job. Thank you for listening!