Baby-sitting will depend very much on personalities, and with such a small age gap you may well end up not with one dc baby-sitting, but with each dc being responsible for themselves (and maybe older dc for the safety of the house)
Can't quite remember, but think dd was taking her younger brother (3 1/2 yr age gap) into town by the time she was 12- more about him baby-sitting her tbh as she had a disability and suffered from anxiety
As for going on holiday, that will depend on how entertaining your holidays are- mine as still happy to come at 16 and 20, and in fact I am about to join my parents for a short holiday at Easter- I'm 53!
We ask the 20yo if she wants to come, but assume that the 16yo will.
Bedroom may depend if different sexes; if so, probably around the time of pre-puberty. If same sex, will depend on issues of space, how well they get on and how well they get on with each other's friends.
I have very much enjoyed mine growing up, not that I didn't like them as babies or toddlers, but there is something very satisfying about seeing them cope with life on their own, or being looked after by a tall lumbering almost-man and know that "I raised this boy to be caring". And you can have interesting discussions with them. That whole thing of seeing their personalities develop- I love it!