My 2 DDs are 14 1/2 months apart. DD1 & DH were with me during her sister's birth (in a hospital, with a doula - I moved around a lot & had no interventions). She got a little worried towards the beginning of the pushing phase, when I was a bit noisy, but DH was there for her, & she watched her sister born & seemed fascinated by this new little person.

We brought her Pack n Play for her to nap & play in, & some toys, stories, & snacks.
Frankly, just going from 0-1 was not easy - DD1 literally could not be set down for about 8 months, & didn't walk until 14 months. Going out of sight - ha! But she was OK with me sitting & reading her things, & playing with her sitting down, when I was heavily pregnant, as long as she could lean on me, or touch me in some way; nursing her sister was just an extension of that, as she was used to me sitting with her (I would have tried tandeming, but she had tapered off a lot & nursed for the last time about 3 days before her sister was born). So going from 1-2 was easy in some ways, hard in others. I couldn't set DD2 down either, as she had severe reflux problems, & preferred sleeping on her tummy upright on my chest; when she was 3 months old, our house burned down in a wildfire, & we moved in with relatives; like some people have said, that 1st year or so with both of them was pretty blurry, but for a lot of reasons!
I didn't actually have them interact with each other a lot, except with
very close supervision, until DD2 was about a year old & starting to walk, just because her sister liked her, but tended to be a little rough with her because she was so little herself. I used a Pack N Play a lot, to set one in while the other buzzed around.
At 2 and a bit, & almost 3 1/2, my DDs are best friends, play together pretty well, & will hug each other & tell her sister she loves her. My oldest does not remember a world in which her sister was not there, & they both pile into my lap, one on each leg, to hear stories. It helps that their personalities are so different; DD1 is cautious, intense, & wants to lead - DD2 is bold, easy-going, & is happy to go along with her sister's ideas.
In some ways, I've wished we'd gone for greater spacing; but I'm also finally getting to the point where I can see that someday, looking back at this time, I think we'll be glad that our 2 are so close.