Great thread. We are beginning to discuss the same question at the Low household. Great advice so far.
Myself, T.Low, has one sibling and I can say it certainly added to my childhood experience. As an adult, we don't get along so well, but it has improved since we learned that my sibling is diagnosed Bipolar and ADHD.
SheLow has two siblings, and I watch the dynamic of the three of them together and it makes me want to have three kids. They are absolutely wonderful together as adults. We won't have three, but I certainly can see the case for giving Olivia a sibling.
On the other hand, SheLow's favorite cousin is an only child and is very close with his parents and cousins and never once wished for a sibling. Same with a close shefriend of SheLow's.
Everything about this experience with #1 has been perfect. Absolutely perfect. The pregnancy, the birth, and the first 14 weeks so far have been better than we ever could have imagined. We have both actually commented that there is simply no way that #2 could be this perfect.
This is a tuff one; but I think ultimately we both want OLow to have a Little Low. And for me, I want SheLow to have some Lows to keep her company in the future as I most likely won't be around as long as her.