Congrats on your wedding!
I haven't been to DR in August (I went in November once and the weather was perfect then although it sprinkled at least a little bit everyday, but before your knew it the sun was out again) but I go to Cuba all the time and they share similar weather. The one time we went in August.. it was unbearably hot!! Like, shower at least twice a day and too hot to suntan type weather. If we were outside, it had to be in the shade and it was counting the seconds til we could be in AC. We were there for 2 weeks that time and in the two weeks, we were hit with Hurricane Ike and Gustavo. The good thing is though that the hurricanes only last for 1-2 days. The rest of the days were perfect and sunny again. We heard that the people in the resorts weren't allowed to leave the hotel those days. If there is a hurricane coming, you'll be able to see it in the weather forecast a bit ahead of time so if one is coming on your wedding day, you could probably just reschedule it to one of the other days that week.
I think everything should be fine! Good luck!