Why does my mom treat me different?

My mom married some guy named Paul that none of her biological kids like (ages 17-11) he makes her penis is for the smallest things like if we forgot to rinse of our dishes etc.. well he has a daughter and idk if it’s just the fact that I have been the only girl for 16 years and not used to sharing my mom , but she treats her way better than she treats me. For example: My mom calls her Baby girl, and says she’s so beautiful and this and that. And my mom doesnt say that to me since she has been around. And it kinda breaks me but whatever. And for her birthday her dad and MY mom went all out and threw the biggest birthday party ever. I got literally nothing for my birthday , now ain’t that some shit. Maybe I’m overreacting but it’s hard to feel good about yourself when your own mom doesn’t even like you lol. (Oh and anything I do so much ask as my mom a question all I get is “Calm down Patricia you are disrespectful” )