I am the eldest of 5 (well 8 if you count step siblings) and as my mom is pregnant there is soon to be another to add to that number.
I never had to worry about hand me downs as I was the oldest, but I always felt I was left out in some ways and I know my sister felt the same. Probably because my brother dies when he was little and the next xhild born was also a boy so my mom was very protective of him and gave him alot more attention than us (which is understandable) My sister and I resented it at times, but it wasnt a huge problem.
We used to argue alot when we were teenages and eventually at 13 or so she moved out to live with our father for a year, by the time she came back I was 16 and living with soon to be DH. We got on loads better after that. The younger children in the family we are more like aunties to (17 year age gaps etc) and when the new baby is born in Feb, there will be a 24 year age gap so I guess it wont really feel like having a new sibling more a neice or nephew.
With regards to the step sisters - I couldnt stand them. Still cant. Half brother is the same age as my youngest brother but I dont see him often enough to really have that brother/sister thing with him (also he is part of my dads family and they are all crackers) It would be nice to know him better, hes a sweet kid.
Growing up in a big family meant noise, no privacy, no space... but also that there was always someone to play with, and we could gang up together when we wanted something or had done something naughty (it wasnt us mom...)
I wouldnt change anything, I had a happy childhood, although we didnt have much money. Looking back i am very impressed that my mom managed to stretch her wage the way she did. I think if I actually looked back to my childhood more and objectivily looked at things I could learn alot about how to be a good parent.