I think hor, just try to talk to you #1 more and let your #1 understand that #2 is on the way. Once they accept that they are gonna have a little brother or little sister, they would usually be more tolerant. Then try to let them know that mummy will have to take care of #2 more next time but mummy still love #1 lor.
Before deliver, get #1 involved in preparing the things for #2. Like packing the clothes, packing your delivery bag. Massaging your back or legs oso can.
Then deliver le, let the #2 share in the responsibility of taking care of #1. Like get them to help you prepare things for #1. When you are bathing/changing #2, ask #1 to help you take things like towel, etc. That way, they feel that they are still important to the family but important in a different way, important in the "I'm an older sibling and have to take care of my sister/brother way".
But hor, the drawback abt this is the #1 might become extremely protective (maybe overprotective) of #2. My cousin (#3) was so protective of her little brother (#4) that when its his bedtime, she will shoo all of us home. Tell us that her brother needs to sleep. Lolx.