Congrats on this big event in your SIL's and DD's lives!
My sister was 3 when I got married. (I was 27.) She started screaming when she couldn't walk in with our mother. She was usually very even tempered and, try as we might, she wouldn't walk in with the other 2 flower girls. I thought she was going to walk in with my sister (MOH,) but at the last minute she decided to walk in right before my MOH. She cried the whole way down the aisle. The video is too funny and I've promised her that I'll walk down the aisle when she gets married, crying, too.
When she got to the front of the church, she sat with our Mom and my SIL has her on video picking apart the flowers in her basket while she was lying on the pew with her head on Mom's lap, knees up in the air so you could see up her dress. Again, this is priceless.
When we were having our first dance at the reception, she also cut in. She was just a typical little girl doing typical little girl things.
My advice is just to be prepared for anything and roll with the punches.
I was in a wedding when I was 4. My uncle's sister was 6. Everyone expected me to misbehave and for her to behave. She became affectionately known as "Crybaby, Chatterbox Christine" after the ceremony. I was a perfect angel. But, because no one expected her to not behave, no one discussed what their expectations of her were. I think this is important. Get your daughter excited, but also discuss what is good behavior during the wedding and what is not. It also helps, if the wedding is in church, that you take your DD to Mass or Services so she knows what to expect. My brother's kids don't go to church ever and when they are forced to go to a ceremonial service, they don't know how to act... they are bored... they make alot of noise.