It is different from stepfamily to stepfamily. There will be problems in trying to adjust. Getting used to stepparents, stepbrothers, and stepsisters is hard and can make you feel lonely or overwhelmed. You might hope your stepfamily can become the “perfect family” right away, but it takes time for everyone to adjust to the new situation. You can always talk to your parents about it as well as a counselor at 1-855-201-2121.
I think that it is very exciting that you have a new baby sister. I remember when my little sister was born I was excited and scared. I was afraid that my mom wouldn't love me as much because of her new baby. What I realized, though, is that parents don't have a certain amount of love they can give to each child. They have endless love for each child. There is no limit to love. Do you love your mom or dad less because you are too busy loving someone else? No. You just love more people. My mom always told my bonus sibs and me when she had my sister that we were helping to raise a child--and you are, too. Be excited and teach her things that you know.
Believe it or not, sometimes my mom gets bugged because now that my little sister is eleven, she would rather hang with my sister and me than my mom! Considering how I felt when she was born, I never would have predicted that!
Find some time to spend with them. If you are the older one, they might think it's extremely cool that you would hang out with them. If you are the younger one, be nice and don't invade your step-brother/sister’s personal space. Stay on your side if you share a room.
Remember that even if it seems impossible, you are family and because of that simple fact, you must have a few things in common. It's always easier to build common ground if you start with what you already have.