We've got 3 years 8 mths between Maya and the gremlins, which altho not planned (we wanted them closer together but it took us over a year to fall pregnant and we lost a baby in between) was perfect - Maya was old enough to understand what was going on, she could be distracted with some crayons or a book if I was feeding/tending to the babies, and now that they are 3 and 6 they play together really well most of the time. We've never had any jealousy issues, they do fight sometimes now but that's a sibling thing and would happen regardless of the age gap.
3 minutes between Sienna and Mercedes - I wouldn't recommend it, if it can be avoided

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20 months between Chiara and the gremlins - it was horrific during my pregnancy as I had hyperemesis and some days was so ill I couldn't stand up (I ended up in hospital) and they weren't walking to start with so I was still carrying them everywhere and having to lift the buggy in and out of the car.
We never had any jealousy issues once lil miss arrived, but the gremlins really had no idea what was going on - they kept going into her room to see if she was still there coz they didn't realise she was ours

. It was exhausting tho as they are really bad sleepers and always have been and I could never seem to get the nap times to coincide. Plus going out was a mission - it was either the gremlins in the buggy and lil miss in the Moby wrap or lil miss in the snap and gos and the gremlins on their monkey backpack leashes.
Now it's awesome tho, it gets easier every day/week/month and they play together really well. They love "teaching" her things, and she is really feisty and stroppy and likes to join in their games which is cool.
I think you have to look at it both long- and short-term - in the short term, a small age gap is bloody hard work, so you have to think about what strategies you can employ to make things easier over that period (support from family/friends, PPL etc.) But longer term, if you can get thru that first few months, it definitely has it's rewards.
We'd definitely be looking at 3 years + if we were to have another, but this shop is definitely closed!