What would you tell this kid?

My dad remarried a while ago. His wife is pregnant, and my sister is devastated. She was very young when my parents got divorced. She’s a teenager now, but still a young one. She’s worried she will get replaced, and I’m sure there’s other stuff playing into it. What would you say/do? My dad is not good at this stuff, and my mom and I are worried.

My sister spends every other weekend at dad’s house.