i've had 5 with varying age gaps and i know your concerns!
#1 and #2 are 2yrs 1mth 2days apart and they're close, #1 was just a lovely little man and never got jealous or agressive of the baby.
when i had #3, #'s 1&2 were already 4&6 and very excited, we had dealt with a miscariage right before we fell pregnant with #3 so they were also aware of that. they even tried to name the baby, lol, thankfully we didnt take to any of the pokemon names offered up!
when i had #4, #3 was only 16mths, and she wasnt overly fussed with him, whenever he cried she would plug him with his dummy and sit with him til i got there, once agian, no real dramas. when i had #5, #3 was 3 and #4 was 21mths.
what i do is basically let them touch and help whenever they want too. babies are very resiliant and wont wake unless really poked or grabbed on. babies also love the new faces to look at. the only time i find my #3 & #4 give me a hard time is when i'm feeding bub, but i let them be involved with that too, and often my 22mth old will climb all over me at the same time.
if u let them be involved and use words like gentle and quietly instead of 'dont' and 'no' u will find they will be great helpers and better with bub.
sahm to 5 midgit circus freaks