What's a good age gap between kids?

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I also have a 9.5 and 7 year gap. The 9.5 is so suboptimal it isn't funny, the two of them have almost zero to do with each other. 7.5 gap was ok until the toddler turned 2.5 and decided she wasn't a plaything anymore, and now the two of them argue constantly, so whenever Miss Almost-10 is home Miss almost-3 is constantly screaming and crying at her big sister, who spends an alarming amount of time expounding on how much she hates her little sister and how she wishes we never had more kids, and the little one has learnt to say her big sister is "stupid", "bored" and "mean", words WE didn't teach her. It is horrible, but also depends on the personality of your kids.

But the two little ones are utterly adorable together when the big one isn't home, at least *sigh*

Edit: the two girls arguing has nothing to do with the baby, the timing is a coincidence. DD#2's personality, expanding vocabulary, capability, independance and will all asserted themselves at about that age and she started standing up for herself, which causes shocking arguments and tantrums with her sister