I need someone to talk to...

Ok so heres a little bit of the back story. I have 2 children from a previous marriage 1 girl Samantha which is 4 years old. 1 boy Walter, 2 years old. Their dad, and I split up over a year ago. Well in March 2018 i started dating my bf. Everything is perfect. I can say he is the love of my life. We also have a baby on the way due in april with a little girl. Hes great to my kids, he really favors my little boy. Which is great and all, but him and his family are constantly getting on to my daughter (samantha). It seems like for every little thing. He has 2 little nieces that come over all the time that dont get into near as much trouble as samantha does. They can do the same exact thing as her and not get in trouble, but she will. My bf's grandma is the worst about it. Ive also started noticing samantha acting out more. Like hitting her brother, and shaking when she does it. We were also in the store today and she called some man fat. She is telling her father that my bf's nieces are mean to her. They are 2 and 3 years old. Im really worried and dont want her feeling like she is beneath anyone. My kids and i will be moving in when the baby is born, and i dont want to if this is just going to get worse. Is anyone else out there going through this?