What will be a good age gap for siblings?

Mine is 3 years apart and I do think it is a good gap; as I noticed my firstborn do understand the concept of being a Jiejie so she often assist me a lot in teaching and guiding her younger sibling in a "children way". My younger is currently under going "horrible" two where she always test my boundary which result in me yelling at her sometimes; time like this my firstborn will step in and guide her, and meimei does listen and behave herself most of the time.