my opinion, 2 ~ 3 year in between will be the best.

Timing is very important (for sibling relationship + time adjustment + personal time ...etc)

One of my brother older than me 12 years, when I went to primary school, he aldy prepare to uni. So our relationship is not so close, but my another brother younger than me 2 years, we are very close and take care of each other (of coz sometime will fight when we were young, but sweet memories now )

Then, my elder brother has their 2nd child right after my 1st niece (means 1 year older than 2nd niece) ... I can still remember the hard time taking care of 2 babies especially midnight, the whole family woke up to take care of them and everyone lack of sleep