If things happen for you like they did for me (and who can tell, everyone is so different), my boobs grew, so hopefully that strapless issue might work out after all! And I sorta rounded out, but didn't properly 'pop' til about 22 weeks.
This is my first too, and it seems to go that you pop later with your first. So you just might be alright! Especially if you have some room to grow into
Perhaps talk with your sister and have a backup plan, just in case your body decides to do its own thing.