There's 10 years between ds1 and dd1, 12 between ds1 and ds2 and 16 between ds1 and dd2. DS1 barely blinked when dd1 was born. Whenever I stopped by his school with the baby in the backpack, he'd steal her and take her into his class to show her off. We asked him to help with little things ("could you grab me a diaper" type things), and let him get as involved as he wanted with her. He used to love getting her yogurt and stuff when she started eating some solids. And, he never seemed to notice that she was getting a lot of "his" attention (and he'd been an only child until that, so he was really, really used to attention). It just wasn't really an issue. I think he felt his sister was just so adorable that it seemed only natural that everyone talked about her, yk? And, he's so proud of super cute dd2 (she has an amazingly engaging personality) that he'd probably look at a person funny if they paid more attention to him than to her. Mind you, ds1 is very good at getting his emotional needs met, and doesn't lack for attention, either at home or at school.
Now that the house is full of "little people" (as ds1 usually refers to them), he gets more frustrated. He's realized in the last year or two that he'd have more material goods if he didn't have so many siblings, and while he's not super materialistic, he is in the position of being one of the lower income kids at his school. His closest friends are very well off, and he does notice the contrast. That said...he wouldn't trade his siblings for anything. Most of his issues with them - and there aren't many - are about the way they treat his stuff. I'm not very sympathetic on most of it, because ds1 has more space to himself than anyone else in the house (dh, dd2 and I all share the bigget room, and dd1 and ds2 share the smallest one - ds1 has the medium bedroom completely to himself). He also has a lock.
He absolutely dotes on all of them, especially dd2. We have had a couple of occasions when people have mistaken him for the dad, which is really...awkward. But, other than that, it's been a remarkably seamless transition from only child to oldest (by a lot) of four.