I found a 3yo and a new baby very hard work. But it only took a few months for them to settle down- and since then we've had 12 years of mainly pros:
coming into their room late at night and finding them snuggled up together because little brother has had a nightmare and big sister has comforted him
being able to cook a meal in peace because they are entertaining each other
seeing big sister grow tall with pride when other children admire her cute little brother
having less time to worry about every single bit of parenting
finding, when little brother starts playschool, that he already knows half the children, because they are siblings or cousins of big sister's friends
little brother having a problem at school and being able to talk to somebody who really knows the place inside out (no, Miss Cross-face isn't really that scary, she just looks that way)
knowing that when you have a bad falling out with one of your children they are still not totally alone- there is someone in the house they can talk to
being able to send them into town on a shopping expedition while I'm putting my feet up at home (ds, how could you let her buy that canary yellow top, what were you thinking of?)
I think the first time it really came together for me was when ds was 6 months old and dd was having her photo taken at play-school. For some reason I missed the note that said that you can bring younger siblings in to have them taken as a group. When I turned up to collect dd she was in floods of tears because she hadn't been photographed with her little brother: I had to take her straight home and dress her and ds in their best and let them pose in the garden. That's when I really felt that "this sibling thing isn't just a thing I have imposed on her, she really loves him and is proud of him".
The last time it was brought home to me was the other week when dd had a potentially serious accident. Ds, who came home and found her, sobbed all the time while ringing emergency service and sat holding her hand in A&E. And again I looked at them and thought "they really really love each other".