I'm the oldest of five kids, four stairsteps and a caboose. I wouldn't say any of us are particularly close, though I can't imagine us getting into some of the hateful fights I've seen some families enter. My kids however are close and to me it is a joy to watch. My son has always been very attached to me, so we were worried about how he'd deal with his new sister. The worries were for naught--he was crazy about her. As his interests become narrower and hers become more broad they don't spend as much time together, but on the other hand, they don't fight. I was at a meeting at school one day discussing my son and someone asked about their relationship. I said that their interaction was as close to normal interaction as he would achieve. They would truly engage in cooperative vs parallel play. Then the speech therapist, who saw them both (she had picked up a few of his articulation problems) said "they are so sweet, and so cute together but definitely NOT normal". Basically he won't fight with her.
The girls too are close, even though they are nine years apart. The older one was on a Girl Scout trip to Washington DC last week and the baby really missed her. She was so thrilled when my husband brought her sister home last night and this morning the two of them were reading and I heard the older one tell the baby "I really missed you".