My hubby is in the middle of 7, I am 3rd of 8, and we have 5 kids. When I was pregnant with my 1st, I started asking mom's how far apart their children were and how they got along with each other. In general, kids who were more than 3 years apart didn't play as much together as kids who were 18 months to 2 1/2 years apart. My kids... child #1 and child #2 are 19 months apart, then it's 4 years to #3, then 2 years to #4, then 4 years to #5.
Having said all of that, how you teach your kids to love each other will be bigger/greater than any age difference. As soon as I start sharing the news that I am pregnant, I try to choose my words carefully with my other kids. If you need a nap or what you are eating or activities that you can't or should do... not 'blaming' it on the baby. "Your going to be such a great brother/sister." "He/she/the baby is really going to love you!" Letting your child even make some choices that don't have to be one way for you, "Which blue blanket, this one, or this one?" (holding up the two that you are deciding between.)
Then when the baby comes, "The baby is crying, do you think she needs a blanket? Do you know where it is? Wow, thanks for bringing her blanket. She ready likes you. Oh, she likes it when you hug her gentally, like this." I had my todlers grabing diapers and toys and clothes for me. Then when they would be too much in the babies face, or too rough, I would say how the baby likes to have them do it.
There was all kinds of fighting with my siblings when I was little, and my mom told me later (I think that it was when I was pregnant with my 1st), that she didn't know that she had to teach us kids to love eachother, she thought that it would just happen since we were siblings (she was an only child). One of the very best pieces of advice I got as a mom. It's made a HUGH difference!
I don't know what your views on life are. We mix and interact with so many people through out our lives. You do not know who your children will meet and what part they will play in the lives of others. They have the potential for great and wonderful things.
Also, not that you asked, but I get asked a lot... for me, adding child #2 to our family was the 'hardest'/biggest adjustment. I think it was because of where we were with our lives and such. I have heard that for some, adding child #3 was harder, but I think that personalities and situation and age difference of the kids all adds to that. It does not 'get that much harder' with each one, just with #2 or #3 child... in general.
I hope that I was able to help in some way.