Daughters vs daughter in law

I'm having a really hard time with my sil. She's 6 weeks ahead of me in her pregnancy and I am due mid April. My sil thinks that my mother should treat her the same way she treats me as in helping me out with some stuff and what not but does nothing on her end to even try to have a relationship with my mom. My mom is so excited to be a grandma but she knows she will only see their baby every once in awhile and she will see mine several times a week if I can make it work. I just don't understand her feeling like my mom is required to treat her like a daughter when she's not even an active member in the family. And to be honest she isn't her daughter, she is her daughter in law, my mom didn't raise her, did not watch her grow up. I'm just sad for my mom because I feel like my sil is trying to just drain my mother dry of money and thinks she's more deserving of my mother's attention and time than I am. Maybe it's the pregnancy??