I have kids aged 11, 10, 8 and 17 mos. Yes, the toddler was a....surprise. I was afraid of that, too. My sister and I are 7 years apart and growing up, we weren't that close. Now, we are very close.
My kids are sooooo in love with their baby sister. I figured my older daughter (8), would be close to her, eventually, too (like my sister and I), but it's my oldest son (11) and the toddler who have a special bond. It's crazy and wonderful.
I won't lie to you and say that the activities are not a problem. My older kids are in football, cheer, baseball, guitar, etc. Yes, it's hard to cart around the baby/toddler. I can't sit and watch them practice or really enjoy their games, recitals, etc. It sucks sometimes. But, if you find a good babysitter to watch the younger kids while you spend time with the older kids, it can work out fine. I think that will be one of the things you'll have to carve out time for. The younger ones will have a different interest than the older ones. For example, if you can't watch your 10 year old's homerun because you are running after toddlers and tending to a baby, then maybe you can find someone to watch them while you concentrate on him. Also, we can't just take off and go to a movie like we used to. We'll pick out a movie everyone wants to see and find a babysitter for our toddler. This way, we are still doing the things we used to, but still have the toddler for the fun things we all enjoy. We recently went to the apple orchard where my toddler had a BLAST picking apples. It was so much fun for all of us to watch her. We also did Disney with her as a 9 month old. Sure, my hubby and I had to take turns doing the rides and such, but the older kids still had so much fun and didn't even seem to notice.
There will be sacrifices made by the family as a whole, but as far as how close they are....only time will tell. The older they get, the more they will find they have in common. Sure, I don't know my youngest brother as well as I'd like(we are 12 years apart), but when we all get together, it's like we have been hanging out everyday for the last year. It will depend on your kids' personalities, too. Don't worry, they'll love each other and be protective. They'll also fight like cats and dogs. It's how siblings were meant to be.........congratulations and have fun with them all!!! :)