My fist was 15 months old when my second was born. I was worried that he would be jealous but actually he was really good. I think he was too young to think about it much, tbh. An older child might start thinking about being 'replaced' etc, but under 2, well, it's about what they see.
Do I brought ds2 home and the first thing I did was go to ds1 and say "Look what mummy has got for you." in a really excited voice.
His face was a picture! He was fascinated. I let him touch, prod and poke away - making sure he was as gentle as possible, but not worrying overmuch - a hard prod is not going to kill the baby! (just watch the eyes!) I talked to him loads about what I was doing, involved him in everything "What are we going to do next?"
I bought him new toys - presents from his brother who loves him so much.
And I ignored the baby quite a lot!! To make a big show of putting ds1 first. Baby had to wait until mummy had helped ds1 etc etc..
All that sort of stuff really helps. But I am glad ds1 was so young when ds2 was born, because they adjust so well at that age, ime. they quickly forget there was a time it was just them, whereas older kids can remember quite clearly when they had mum and dad to themselves - before the interloper arrived!
Oh, I also told everyone in advance that when they came to see the new baby they had to ignore him at first, go straight to ds1 and make a fuss of him before saying "I hear you've got a baby brother. Can we have a look at him?"
We found the problem time was not right away, but when ds2 got mobile and started getting into ds1's stuff. But by then he couldn't remember a time before ds2 and I just let them fight it out. Worst thing you can do is routinely interfere. (step in though for bullying etc) They found their way through it all.
Fast forward 7 years and they fight like cat and dog and make more noise than a herd of elephants and the mess - oh the humanity... In other words - Perfectly normal siblings, who love each other to bits and benefit from being close in age because they have each other to play with (fight with).