Is a 1 or 2 grade gap better between kids?

We have an early August birthday and a March birthday. Were trying to decide if we should start our first daughter in school at 5 or hold her back a year until she's 6 but then theres only be a 1 grade vs 2 grade difference between her and her sister. I know the pros for kids waiting until they are older vs starting young but can't find much on the sibling dynamic of having a 1 or 2 grade difference. Is there more competition? Are they closer? Do they have the same friends? If anyone has experience either way thatd be greatly appreciated.

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