I decided to have more children for exactly that reason, and like you, felt weird and conflicted by it. My 2 ds's are now 28 months and 9 months and already have an incredible relationship. I could never have guessed how cool it is to watch my kids together. My dh and I always talk about how it's more than we imagined. My 2 year old wakes up every morning and says "Ma, where's my baby?" and Gavin instantly shrieks with delight. Then they play all day together.
I guess my rambling point is even though that might be your only incentive right now, you certainly won't regret it and will realize how wonderful it is after and your children will only benefit from having someone to go through life with.
Incidentally, I'm an only child who never once wished I had a sibling. I grew up in a divorced home but had lots of cousins and sibling-like relationships as well. I wanted only one child but (oops!!) now we're on #3
Good luck to you!!
Mom to Brennan (10/00), Gavin (5/02), wife to Andrew and due in July!!