Dear Melanie,
Having a new person in the family always means an adjustment. And for you, you have to get used to three new people, which is particularly hard. You're probably feeling confused because you want your mom to be happy, but at the same time you're a little jealous that she is giving time and attention to other people, including your step-siblings. Give yourself time to get used to this new family, work at accepting them (it might help to remind yourself about how important this is to your mom), and when you need to, talk to your mom about your feelings.