Our girls were 22 mos, adjusted age 19 1/2 mos, for their first trip. We stayed at OKW and having the washer/dryer was a godsend. We took things at their pace. My girls were never early risers, still aren't, so we would just go when we were all ready, and play it by ear. There are so many great things in MGM and AK for smaller kids, that you may spend a fair amount of time in those parks as well.
As for Sea World, at that age I wouldn't go. There really isn't that much there for them to do, other than a couple of the shows, and play area. Plus, you're about a 20 - 30 minute drive back to SSR from there.
My girls were 2lbs13oz and 3lbs8oz at birth. Other than some sensory integration issues and a minor LD for one of them, the girls are great and are now 12 entering 7th grade. I will share with you the best advice I got regarding twins, and that was from my pediatrician, also the father of twins, never let them out of the stroller!
Your DVC purchase will be one you will enjoy as well. We own at BWV and OKW. We have 3 dds, so the 2 bdrm for us is great. Only bad thing is we are truly spoiled and now try to only stay in timeshares when we travel.