Hmmm…
I imagine first they’d be spaced out about 18-24 months each. So Acacia would be 4 and Hank 2 when Willow is born.
They’d still be close, closer than most siblings are, because of who they are, their home life, and the unique circumstances of their childhood. But I think birth order would definitely be more emphasized here.
So Acacia’s bossiness would be taken up a notch because she’s the oldest, and she gets to go to bed at 9 when Willow has to go to bed at 8 and you should listen to her darnit. The slight babying Willow gets that drives her nuts already is even more pronounced when she is the LITERAL baby of the family. And if you thought Hank was the ultimate middle child in regular AU; HOO BOY.