We took my lb along to the scans (he was 1 and a half when I became pregnant and just over two when they were born) and he loved seeing the 'baby heads' on the screen. We used to talk lots about the babies in mummy's tummy and what he thought their names would be. We also talked about the fact that babies can't use words so they would cry a lot and would be too small to hold and play with any toys for awhile. He helped to choose their car seats and the decorations for their room.
I was induced so we knew they day they would arrive, so he knew that mummy and daddy would be coming home with the babies. My DH and ILs gave him a 'best big brother' t-shirt and brought him along to the hospital to see his little sisters. He was so excited that he wasn't bothered about seeing me! When we brought the babies home we picked him up from the ILs on the way so he could 'help'.
We tried to keep his routine the same as before so he still has his one day a week at the ILs and two mornings at nursery. The babies sleep more in the mornings so that's when I try to have some one to one time with him. We do painting, or make cakes, or if the weather is nice I bundle the twins into the double pram, he sits on the footrest at the front and we walk to the park. I let him make the decisions about which nappies they will wear (we use cloth) and which clothes to put on them. When they used to pull their windy faces I would pretend they were having a funny faces competition and he could decide the winner. He likes to help make their bottles of formula (his job is to shake the bottles) and he always wants to help wash up the empty bottles afterwards. Now he is a little older he sometimes helps me feed them. He also likes the job of wiping their milky dribbles with the muslin. It helps to make caring for the babies into a game for him.
In they early days I did have to ask the ILs to have him for the odd extra day. I also asked them not to talk about the babies with him when he was there, just to give him a break. You'll get lots of attention when you are out and about from random strangers. If DS was with me I'd also mention what a fantastic brother he is so some of the attention was directed towards him too.
He still naps in the afternoon so this is my time with the babies. To begin with they just slept on me on the sofa, five months on they like to roll on their mat and it's nice to have some uninterrupted time with them.
I can't say that he wasn't affected by the arrival of the twins. He needed more cuddles and reassurance, and there were times when he just had to wait whilst I sorted out the babies. He does also watch more tv than before, Netflix has been a bit of a life saver. I think this extra neediness lasted about 3 months before we got our happy, confident little boy back.
I would also recommend getting your DD used to playing on her own for short periods, and you may have to be a bit more relaxed about things you'd normally stop her from doing. If my ds decides to empty all the DVDs off the shelf for example I'm now fine with that as at least he is entertained for a while!
Hope this gives you some ideas.