Question for Mom's with an older kid: How do you get your other child used to having to share? My SO…

Question for Mom's with an older kid: How do you get your other child used to having to share? My SO has a 3 almost 4 yr old daughter who is very much used to being the center of everything. Every thing in the house and all the people and pets are only hers. She's already getting put out by all the baby stuff we're (slowly) getting and starting to get temperamental and more possessive. Tried explaining it, and she's thinking it'll be another friend like her to play with. We've tried getting her little things too, clothes, toys, upgrade to a booster seat and even upgraded her to a full twin bed set. She still wants to have the baby stuff, to ride in the stroller, sleep in the crib and use all the baby toys, soap and other things. Now I had someone give me a Minnie Mouse baby blanket, which is adorable and the only issue is that Minnie is HER favorite so now she wants it. Telling her no results in 2 days straight of fits, timeouts and her not talking to us again. (She has a speech impediment from being a premie and started talking late as is.) Its exhausting as she usually only needs a time out maybe once a week and suddenly it's multiple a day because she won't share! I don't know what I'm going to do when I give birth and can't give her all the attention anymore.