I remember when I got pregnant the second time, we were wondering how we would survive two kids... but children do change so much in those first 2-3 years, that by the time his sister came, he was very different than when I first knew I was pregnant 8 months earlier. And because they are in different phases of development now, it does not seem as daunting as it did before.
Some things are easier the second time around just from having some experience - like we already knew how to diaper (the amount of mishaps from poor diaper changes dropped dramatically the second time, and I don't think we called the on call doctor for the same set of issues as much the second time around when something freaked us out).
There are definitely no guarantees that siblings will be friends (I have a sister with such intense sibling rivalry to the point that she went ballistic when my mother accidentally mentioned my daughter's name to her - so we don't have any contact) but right now, our kids squabble over the usual stuff, and like to play with the same toys (luckily for DS, DD does like trains and trucks along with dolls and butterflies).
It is reassuring to me to know that others with gifted kids do co-sleep and nurse until later since it seems like everyone else around us have kids sleeping through the night in separate rooms by 6-9 months and weaned at a year (DS cut off nursing himself by 16 months and he switched to playing with my then-growing belly but DD is 23 months and shows no sign of wanting to stop - in fact, she uses it to claim her time with me and works very hard at times to kick DS away while she nurses).
