Gorgeous dress, but you definitely need some shapewear.
You can actually get shapewear of lots of different types in TK Maxx - I've bought lots of bits from there so check it out - they're super cheap.
From a technical point of view; the reason why your tummy looks more pronounced is the cut of the bottom of the bodice area. This could easily rectified by making the bottom of the bodice area asymmetrical instead of pointy at the middle. If it's asymmetrical, the eye is drawn to the smallest part (your waist) rather than the widest part (where the point bit is). If you wanted to ask your brother to alter the dress, this is the part that I would ask him to focus on.
The dress is amazing though, but perhaps you'd be happier if you could see yourself in other dresses too so you know what actually works for you.
Good Luck.
Tx