Everyone I know that has a good relatioship with their sibling(s) is about 14-20months different. They grew up close and remain close. I am three years older than my brother and seven years older than my sister and it has only been since we are adults that we have got along. Probably the boy girl thing when we were younger and I was fourteen when my sister was seven - not a lot in common.

Your body needs at least 12 months to recover from your first pregnancy (to help the second baby get enough nutrients etc).

I had a 2 year gap between my boys and this has been perfect.  Enough time to breastfeed and care for the first baby and prepare for a second.

Some of my friends have a much smaller gap (eg 2 babies in less than 12 month).  This has impacted on their breastfeeding - the milk changes and their first baby stopped feeding. It is also hard as the first baby still needs lots of attention. Now that some of their kids are older they seem to fight a bit too (may or may not be due to age gap).

Some people I know have an 8 year gap. This would be hard as you would need to re-learn all the baby stuff again and they probably won't be as close (as friends) until they are adults.

There are lots of pros and cons but ultimatley it is up to nature. It can be hard to plan exactly you may not fall pregnant easity or sometimes you end up with an unexpected surprise, either way, it all will work out in the end.

There are lots of websites that have more info - just do a search for sibling gaps.  Good luck